diff --git a/packages/charts/src/components/bar-chart/bar-chart.tsx b/packages/charts/src/components/bar-chart/bar-chart.tsx
index 5acf99c452..5b9f7fe4cd 100644
--- a/packages/charts/src/components/bar-chart/bar-chart.tsx
+++ b/packages/charts/src/components/bar-chart/bar-chart.tsx
@@ -3,6 +3,7 @@ import type { ComponentPropsWithoutRef } from 'react'
 import { forwardRef, useId, useMemo } from 'react'
 
 import { ChartContainer } from '../../internal/chart-container'
+import { chartMessages, type ChartMessages } from '../../internal/messages'
 import type { ChartTokens } from '../../internal/theme'
 import type {
   BarChartDirection,
@@ -10,10 +11,9 @@ import type {
   BarChartSeries,
 } from './bar-option'
 import { buildBarOption, defaultMaxSeries } from './bar-option'
-import { messages } from './messages'
 import './register'
 
-const useMessage = createMessageHook(messages)
+const useMessage = createMessageHook(chartMessages)
 
 /**
  * Bar charts compare values across categories, in the Shoreline design
@@ -149,15 +149,4 @@ export type BarChartProps = BarChartOptions & ComponentPropsWithoutRef<'div'>
 /**
  * Bar chart internal messages
  */
-export type BarChartMessages = Partial<{
-  [key in BarChartMessagesKeys]: string
-}>
-
-/**
- * Bar chart internal messages intl keys
- */
-type BarChartMessagesKeys =
-  /** Rendered in place of the chart when there is no data to display */
-  | 'empty'
-  /** Names the series that aggregates the tail of `series` */
-  | 'others'
+export type BarChartMessages = ChartMessages
diff --git a/packages/charts/src/components/bar-chart/bar-option.ts b/packages/charts/src/components/bar-chart/bar-option.ts
index 93ec547b9d..cfc44b908d 100644
--- a/packages/charts/src/components/bar-chart/bar-option.ts
+++ b/packages/charts/src/components/bar-chart/bar-option.ts
@@ -1,11 +1,16 @@
 import type { EChartsCoreOption } from '../../internal/echarts'
 import type { ChartTokens } from '../../internal/theme'
-import { chartSeriesTokens } from '../../internal/theme'
 import type { ChartTooltipDelta } from '../../internal/tooltip'
+import type { ChartAxisPointer } from '../../internal/option'
 import {
-  createAxisTooltipFormatter,
-  createTooltipPositioner,
-} from '../../internal/tooltip'
+  buildAxisTooltip,
+  buildCategoryAxis,
+  buildGrid,
+  buildLegend,
+  buildValueAxis,
+} from '../../internal/option'
+import type { ChartSeries } from '../../internal/series'
+import { collapseSeries, createDeltaLookup } from '../../internal/series'
 
 /**
  * A single bar series.
@@ -53,67 +58,11 @@ export interface BuildBarOptionArgs {
   tokens: ChartTokens
 }
 
-/**
- * Hard ceiling on rendered series, set by the palette itself: there are only
- * this many designed colors and they are never cycled, so no chart can show
- * more distinct series than this regardless of what `maxSeries` asks for.
- */
-export const seriesLimit = chartSeriesTokens.length
-
-/**
- * Default number of rendered series: the primary and secondary series plus the
- * aggregate in the tertiary color.
- */
-export const defaultMaxSeries = 3
-
-/**
- * Folds the tail of `series` into a single aggregate so that no more than
- * `maxSeries` series render, keeping every value represented instead of
- * dropping data. Requests above `seriesLimit` clamp to it, because past that
- * point the palette has no color left to tell series apart.
- *
- * The aggregate takes the last rendered slot, so it wears that slot's color —
- * the tertiary color at the default `maxSeries`.
- *
- * Values are summed per category. A null contributes nothing, and a category
- * where every folded series is null stays null, so the aggregate renders no
- * bar rather than a spurious zero.
- *
- * The aggregate carries no deltas: they arrive already formatted, so there is
- * no sound way to combine those of the series it replaces. Series that keep
- * their own slot keep their own deltas.
- */
-export function collapseSeries(
-  series: BarChartSeries[],
-  othersLabel: string,
-  maxSeries: number
-): BarChartSeries[] {
-  // A fractional or non-finite request would otherwise slice unpredictably.
-  const limit = Math.min(Math.max(Math.floor(maxSeries) || 1, 1), seriesLimit)
-
-  if (series.length <= limit) return series
-
-  // The aggregate occupies the last slot, so one fewer series keeps its name.
-  const rest = series.slice(limit - 1)
-  const length = Math.max(...rest.map((item) => item.data.length))
-
-  const data = Array.from({ length }, (_, index) =>
-    rest.reduce<number | null>((sum, item) => {
-      const value = item.data[index]
-
-      return value === null || value === undefined ? sum : (sum ?? 0) + value
-    }, null)
-  )
-
-  return [...series.slice(0, limit - 1), { name: othersLabel, data }]
-}
+// Re-exported so the package's public `defaultMaxSeries` / `seriesLimit`
+// exports keep flowing from the same path they always have.
+export { defaultMaxSeries, seriesLimit } from '../../internal/series'
 
 const radiusToken = '--sl-radius-1'
-const spaceSmall = '--sl-space-2'
-const spaceLegend = '--sl-space-10'
-// Design spec, positioning: 8px between the hovered bar and the tooltip
-const tooltipOffsetToken = '--sl-space-2'
-const tooltipOffsetFallback = 8
 // Design spec, behaviour: hover overlay over the selected category
 const hoverOverlayToken = '--sl-bg-muted-plain-hover'
 
@@ -151,7 +100,7 @@ function barBorderRadius(args: {
  * their own end.
  */
 function isStackEnd(args: {
-  series: BarChartSeries[]
+  series: ChartSeries[]
   seriesIndex: number
   categoryIndex: number
   grouping: BarChartGrouping
@@ -197,59 +146,28 @@ export function buildBarOption(args: BuildBarOptionArgs): EChartsCoreOption {
   // the first series where the reader starts.
   const reverseTooltipRows = direction === 'vertical' && grouping === 'stacked'
 
-  // Deltas are looked up by the series' position, not its name: names are not
-  // required to be unique, and two series sharing one would otherwise both
-  // resolve to whichever came last. The collapsed list is exactly the order
-  // the engine receives its series in, so the index it reports lines up here.
-  // Left undefined when no series supplied deltas, so the tooltip skips the
-  // lookup entirely for the common case.
-  const deltasBySeries = series.map((item) => item.deltas)
-
-  const getDelta = deltasBySeries.some(Boolean)
-    ? (seriesIndex: number, categoryIndex: number) =>
-        deltasBySeries[seriesIndex]?.[categoryIndex] ?? undefined
-    : undefined
-
-  const categoryAxis = { type: 'category', data: categories }
-  const valueAxis = { type: 'value' }
+  const axisPointer: ChartAxisPointer = {
+    type: 'shadow',
+    shadowStyle: { color: tokens.get(hoverOverlayToken) },
+  }
 
   return {
-    legend: { show: showLegend, left: 0, bottom: 0 },
-    tooltip: {
-      trigger: 'axis',
-      axisPointer: {
-        type: 'shadow',
-        shadowStyle: { color: tokens.get(hoverOverlayToken) },
-      },
-      // The tooltip is allowed to overflow the chart. `confine` would push it
-      // back inside the chart bounds, and leaving the element inside the chart
-      // container lets any `overflow: hidden` ancestor clip it — so it renders
-      // on `body` instead, above whatever the chart sits in.
-      confine: false,
-      appendTo: 'body',
-      // Visuals come entirely from the formatter's own markup + tooltip.css
-      // (data-sl-chart-tooltip*), not from the engine's tooltip container.
-      backgroundColor: 'transparent',
-      borderWidth: 0,
-      padding: 0,
-      extraCssText: 'box-shadow: none;',
-      formatter: createAxisTooltipFormatter({
-        reverse: reverseTooltipRows,
-        getDelta,
-      }),
-      position: createTooltipPositioner(
-        tokens.px(tooltipOffsetToken) ?? tooltipOffsetFallback
-      ),
-    },
-    grid: {
-      containLabel: true,
-      left: tokens.px(spaceSmall),
-      right: tokens.px(spaceSmall),
-      top: tokens.px(spaceSmall),
-      bottom: tokens.px(showLegend ? spaceLegend : spaceSmall),
-    },
-    xAxis: direction === 'vertical' ? categoryAxis : valueAxis,
-    yAxis: direction === 'vertical' ? valueAxis : categoryAxis,
+    legend: buildLegend(series.length),
+    tooltip: buildAxisTooltip({
+      tokens,
+      axisPointer,
+      reverse: reverseTooltipRows,
+      getDelta: createDeltaLookup(series),
+    }),
+    grid: buildGrid({ tokens, showLegend }),
+    xAxis:
+      direction === 'vertical'
+        ? buildCategoryAxis(categories)
+        : buildValueAxis(),
+    yAxis:
+      direction === 'vertical'
+        ? buildValueAxis()
+        : buildCategoryAxis(categories),
     series: series.map((item, seriesIndex) => ({
       name: item.name,
       type: 'bar',
diff --git a/packages/charts/src/components/bar-chart/messages/index.ts b/packages/charts/src/components/bar-chart/messages/index.ts
deleted file mode 100644
index 7093d13cdd..0000000000
--- a/packages/charts/src/components/bar-chart/messages/index.ts
+++ /dev/null
@@ -1,27 +0,0 @@
-import bg from './bg.json'
-import de from './de.json'
-import en from './en.json'
-import es from './es.json'
-import fr from './fr.json'
-import it from './it.json'
-import ja from './ja.json'
-import ko from './ko.json'
-import nl from './nl.json'
-import pt from './pt.json'
-import ro from './ro.json'
-import th from './th.json'
-
-export const messages = {
-  'en-US': en,
-  'es-AR': es,
-  'fr-FR': fr,
-  'pt-BR': pt,
-  'ja-JP': ja,
-  'ko-KR': ko,
-  'it-IT': it,
-  'nl-NL': nl,
-  'ro-RO': ro,
-  'bg-BG': bg,
-  'th-TH': th,
-  'de-DE': de,
-}
diff --git a/packages/charts/src/components/bar-chart/messages/bg.json b/packages/charts/src/internal/messages/bg.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/bg.json
rename to packages/charts/src/internal/messages/bg.json
diff --git a/packages/charts/src/components/bar-chart/messages/de.json b/packages/charts/src/internal/messages/de.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/de.json
rename to packages/charts/src/internal/messages/de.json
diff --git a/packages/charts/src/components/bar-chart/messages/en.json b/packages/charts/src/internal/messages/en.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/en.json
rename to packages/charts/src/internal/messages/en.json
diff --git a/packages/charts/src/components/bar-chart/messages/es.json b/packages/charts/src/internal/messages/es.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/es.json
rename to packages/charts/src/internal/messages/es.json
diff --git a/packages/charts/src/components/bar-chart/messages/fr.json b/packages/charts/src/internal/messages/fr.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/fr.json
rename to packages/charts/src/internal/messages/fr.json
diff --git a/packages/charts/src/internal/messages/index.ts b/packages/charts/src/internal/messages/index.ts
new file mode 100644
index 0000000000..f4cc519c48
--- /dev/null
+++ b/packages/charts/src/internal/messages/index.ts
@@ -0,0 +1,46 @@
+import bg from './bg.json'
+import de from './de.json'
+import en from './en.json'
+import es from './es.json'
+import fr from './fr.json'
+import it from './it.json'
+import ja from './ja.json'
+import ko from './ko.json'
+import nl from './nl.json'
+import pt from './pt.json'
+import ro from './ro.json'
+import th from './th.json'
+
+/**
+ * Shared chart internal messages, localized from the surrounding
+ * `LocaleProvider`. Every chart uses the same set: the empty-state label and
+ * the name of the series that aggregates the tail past `maxSeries`.
+ */
+export const chartMessages = {
+  'en-US': en,
+  'es-AR': es,
+  'fr-FR': fr,
+  'pt-BR': pt,
+  'ja-JP': ja,
+  'ko-KR': ko,
+  'it-IT': it,
+  'nl-NL': nl,
+  'ro-RO': ro,
+  'bg-BG': bg,
+  'th-TH': th,
+  'de-DE': de,
+}
+
+/**
+ * Chart internal messages intl keys
+ */
+export type ChartMessagesKeys =
+  /** Rendered in place of the chart when there is no data to display */
+  | 'empty'
+  /** Names the series that aggregates the tail of `series` */
+  | 'others'
+
+/**
+ * Chart internal messages
+ */
+export type ChartMessages = Partial<Record<ChartMessagesKeys, string>>
diff --git a/packages/charts/src/components/bar-chart/messages/it.json b/packages/charts/src/internal/messages/it.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/it.json
rename to packages/charts/src/internal/messages/it.json
diff --git a/packages/charts/src/components/bar-chart/messages/ja.json b/packages/charts/src/internal/messages/ja.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/ja.json
rename to packages/charts/src/internal/messages/ja.json
diff --git a/packages/charts/src/components/bar-chart/messages/ko.json b/packages/charts/src/internal/messages/ko.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/ko.json
rename to packages/charts/src/internal/messages/ko.json
diff --git a/packages/charts/src/components/bar-chart/messages/nl.json b/packages/charts/src/internal/messages/nl.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/nl.json
rename to packages/charts/src/internal/messages/nl.json
diff --git a/packages/charts/src/components/bar-chart/messages/pt.json b/packages/charts/src/internal/messages/pt.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/pt.json
rename to packages/charts/src/internal/messages/pt.json
diff --git a/packages/charts/src/components/bar-chart/messages/ro.json b/packages/charts/src/internal/messages/ro.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/ro.json
rename to packages/charts/src/internal/messages/ro.json
diff --git a/packages/charts/src/components/bar-chart/messages/th.json b/packages/charts/src/internal/messages/th.json
similarity index 100%
rename from packages/charts/src/components/bar-chart/messages/th.json
rename to packages/charts/src/internal/messages/th.json
diff --git a/packages/charts/src/internal/option/index.ts b/packages/charts/src/internal/option/index.ts
new file mode 100644
index 0000000000..add27fba9e
--- /dev/null
+++ b/packages/charts/src/internal/option/index.ts
@@ -0,0 +1,8 @@
+export type { ChartAxisPointer } from './option'
+export {
+  buildAxisTooltip,
+  buildCategoryAxis,
+  buildGrid,
+  buildLegend,
+  buildValueAxis,
+} from './option'
diff --git a/packages/charts/src/internal/option/option.ts b/packages/charts/src/internal/option/option.ts
new file mode 100644
index 0000000000..49bc1cc5a6
--- /dev/null
+++ b/packages/charts/src/internal/option/option.ts
@@ -0,0 +1,103 @@
+import type { ChartTokens } from '../theme'
+import type { ChartTooltipDelta } from '../tooltip'
+import { createAxisTooltipFormatter, createTooltipPositioner } from '../tooltip'
+
+// Shared design-token constants used by every chart's option.
+const spaceSmall = '--sl-space-2'
+const spaceLegend = '--sl-space-10'
+// Design spec, positioning: 8px between the hovered point and the tooltip
+const tooltipOffsetToken = '--sl-space-2'
+const tooltipOffsetFallback = 8
+
+/**
+ * The axis pointer shape our charts use: a shadow band (bar) or a guide line
+ * (line/area). Typed narrowly so the engine's option union doesn't leak into
+ * the shared builders.
+ */
+export type ChartAxisPointer =
+  | { type: 'shadow'; shadowStyle?: { color?: string } }
+  | { type: 'line'; lineStyle?: { color?: string } }
+
+/**
+ * Legend shown only for multiple series, bottom left, per the design spec.
+ */
+export function buildLegend(seriesCount: number) {
+  return { show: seriesCount > 1, left: 0, bottom: 0 }
+}
+
+/**
+ * Chart grid with label containment and token-driven padding. The bottom
+ * padding grows to make room for the legend when one is shown.
+ */
+export function buildGrid(args: { tokens: ChartTokens; showLegend: boolean }) {
+  const { tokens, showLegend } = args
+
+  return {
+    containLabel: true,
+    left: tokens.px(spaceSmall),
+    right: tokens.px(spaceSmall),
+    top: tokens.px(spaceSmall),
+    bottom: tokens.px(showLegend ? spaceLegend : spaceSmall),
+  }
+}
+
+/**
+ * Category axis carrying the chart's labels in render order.
+ */
+export function buildCategoryAxis(categories: string[]) {
+  return { type: 'category', data: categories }
+}
+
+/**
+ * Value axis for the numeric dimension.
+ */
+export function buildValueAxis() {
+  return { type: 'value' }
+}
+
+/**
+ * Shared axis-trigger tooltip envelope: the formatter, position, and DOM
+ * rendering are the same across charts; only the axis pointer (shadow vs line)
+ * and the row order vary, so those are the caller's to supply. Visuals come
+ * entirely from the formatter's markup + `tooltip.css`
+ * (`data-sl-chart-tooltip*`), not from the engine's tooltip container.
+ */
+export function buildAxisTooltip(args: {
+  tokens: ChartTokens
+  axisPointer: ChartAxisPointer
+  /**
+   * Lists the rows in reverse series order. Set it when the chart paints later
+   * series before earlier ones along the reader's axis.
+   * @default false
+   */
+  reverse?: boolean
+  /**
+   * Resolves a row's delta from the position of its series and category.
+   * @default undefined
+   */
+  getDelta?: (
+    seriesIndex: number,
+    dataIndex: number
+  ) => ChartTooltipDelta | undefined
+}) {
+  const { tokens, axisPointer, reverse, getDelta } = args
+
+  return {
+    trigger: 'axis',
+    axisPointer,
+    // The tooltip is allowed to overflow the chart. `confine` would push it
+    // back inside the chart bounds, and leaving the element inside the chart
+    // container lets any `overflow: hidden` ancestor clip it — so it renders
+    // on `body` instead, above whatever the chart sits in.
+    confine: false,
+    appendTo: 'body',
+    backgroundColor: 'transparent',
+    borderWidth: 0,
+    padding: 0,
+    extraCssText: 'box-shadow: none;',
+    formatter: createAxisTooltipFormatter({ reverse, getDelta }),
+    position: createTooltipPositioner(
+      tokens.px(tooltipOffsetToken) ?? tooltipOffsetFallback
+    ),
+  }
+}
diff --git a/packages/charts/src/internal/option/tests/option.test.ts b/packages/charts/src/internal/option/tests/option.test.ts
new file mode 100644
index 0000000000..5dfb184450
--- /dev/null
+++ b/packages/charts/src/internal/option/tests/option.test.ts
@@ -0,0 +1,83 @@
+import { describe, expect, test } from 'vitest'
+
+import type { ChartTokens } from '../../theme'
+import {
+  buildAxisTooltip,
+  buildCategoryAxis,
+  buildGrid,
+  buildLegend,
+  buildValueAxis,
+} from '../option'
+
+const tokens: ChartTokens = {
+  get: () => undefined,
+  px: (token) => (token === '--sl-space-2' ? 8 : 40),
+}
+
+describe('buildLegend', () => {
+  test('hides for a single series, shows bottom left for multiple', () => {
+    expect(buildLegend(1)).toEqual({ show: false, left: 0, bottom: 0 })
+    expect(buildLegend(2)).toEqual({ show: true, left: 0, bottom: 0 })
+  })
+})
+
+describe('buildGrid', () => {
+  test('contains labels and pads from tokens', () => {
+    expect(buildGrid({ tokens, showLegend: false })).toEqual({
+      containLabel: true,
+      left: 8,
+      right: 8,
+      top: 8,
+      bottom: 8,
+    })
+  })
+
+  test('grows the bottom padding to fit the legend', () => {
+    const grid = buildGrid({ tokens, showLegend: true })
+
+    expect(grid.bottom).toBe(40)
+    expect(grid.top).toBe(8)
+  })
+})
+
+describe('buildCategoryAxis / buildValueAxis', () => {
+  test('category axis carries the labels, value axis is plain', () => {
+    expect(buildCategoryAxis(['Jan', 'Feb'])).toEqual({
+      type: 'category',
+      data: ['Jan', 'Feb'],
+    })
+    expect(buildValueAxis()).toEqual({ type: 'value' })
+  })
+})
+
+describe('buildAxisTooltip', () => {
+  test('builds an axis-trigger tooltip envelope around the given axis pointer', () => {
+    const tooltip = buildAxisTooltip({
+      tokens,
+      axisPointer: { type: 'line', lineStyle: { color: '#ccc' } },
+    })
+
+    expect(tooltip.trigger).toBe('axis')
+    expect(tooltip.axisPointer).toEqual({
+      type: 'line',
+      lineStyle: { color: '#ccc' },
+    })
+    expect(tooltip.confine).toBe(false)
+    expect(tooltip.appendTo).toBe('body')
+    expect(tooltip.backgroundColor).toBe('transparent')
+    expect(typeof tooltip.formatter).toBe('function')
+    expect(typeof tooltip.position).toBe('function')
+  })
+
+  test('passes the shadow axis pointer through unchanged', () => {
+    const tooltip = buildAxisTooltip({
+      tokens,
+      axisPointer: { type: 'shadow', shadowStyle: { color: '#eee' } },
+    })
+
+    expect(tooltip.axisPointer).toEqual({
+      type: 'shadow',
+      shadowStyle: { color: '#eee' },
+    })
+  })
+})
diff --git a/packages/charts/src/internal/series/index.ts b/packages/charts/src/internal/series/index.ts
new file mode 100644
index 0000000000..f373527d14
--- /dev/null
+++ b/packages/charts/src/internal/series/index.ts
@@ -0,0 +1,7 @@
+export type { ChartSeries } from './series'
+export {
+  collapseSeries,
+  createDeltaLookup,
+  defaultMaxSeries,
+  seriesLimit,
+} from './series'
diff --git a/packages/charts/src/internal/series/series.ts b/packages/charts/src/internal/series/series.ts
new file mode 100644
index 0000000000..8786378846
--- /dev/null
+++ b/packages/charts/src/internal/series/series.ts
@@ -0,0 +1,94 @@
+import type { ChartTooltipDelta } from '../tooltip'
+import { chartSeriesTokens } from '../theme'
+
+/**
+ * The series shape every chart shares: a named array of values with optional
+ * tooltip deltas. Each public chart owns its own series type
+ * (`BarChartSeries`, `LineChartSeries`, …) with its own JSDoc, and each is
+ * structurally compatible with this one so the shared logic below applies to
+ * all of them.
+ */
+export interface ChartSeries {
+  name: string
+  data: Array<number | null>
+  deltas?: Array<ChartTooltipDelta | null>
+}
+
+/**
+ * Hard ceiling on rendered series, set by the palette itself: there are only
+ * this many designed colors and they are never cycled, so no chart can show
+ * more distinct series than this regardless of what `maxSeries` asks for.
+ */
+export const seriesLimit = chartSeriesTokens.length
+
+/**
+ * Default number of rendered series: the primary and secondary series plus the
+ * aggregate in the tertiary color.
+ */
+export const defaultMaxSeries = 3
+
+/**
+ * Folds the tail of `series` into a single aggregate so that no more than
+ * `maxSeries` series render, keeping every value represented instead of
+ * dropping data. Requests above `seriesLimit` clamp to it, because past that
+ * point the palette has no color left to tell series apart.
+ *
+ * The aggregate takes the last rendered slot, so it wears that slot's color —
+ * the tertiary color at the default `maxSeries`.
+ *
+ * Values are summed per category. A null contributes nothing, and a category
+ * where every folded series is null stays null, so the aggregate renders no
+ * bar rather than a spurious zero.
+ *
+ * The aggregate carries no deltas: they arrive already formatted, so there is
+ * no sound way to combine those of the series it replaces. Series that keep
+ * their own slot keep their own deltas.
+ */
+export function collapseSeries(
+  series: ChartSeries[],
+  othersLabel: string,
+  maxSeries: number
+): ChartSeries[] {
+  // A fractional or non-finite request would otherwise slice unpredictably.
+  const limit = Math.min(Math.max(Math.floor(maxSeries) || 1, 1), seriesLimit)
+
+  if (series.length <= limit) return series
+
+  // The aggregate occupies the last slot, so one fewer series keeps its name.
+  const rest = series.slice(limit - 1)
+  const length = Math.max(...rest.map((item) => item.data.length))
+
+  const data = Array.from({ length }, (_, index) =>
+    rest.reduce<number | null>((sum, item) => {
+      const value = item.data[index]
+
+      return value === null || value === undefined ? sum : (sum ?? 0) + value
+    }, null)
+  )
+
+  return [...series.slice(0, limit - 1), { name: othersLabel, data }]
+}
+
+/**
+ * Builds a positional delta lookup from a series list: the tooltip calls it
+ * with the series and category positions the engine reports, and it returns
+ * the delta for that data point or `undefined` when there is nothing to
+ * compare against.
+ *
+ * Returns `undefined` entirely when no series carries deltas, so the tooltip
+ * skips the lookup for the common case. Identity is positional rather than by
+ * name, because names are not required to be unique and two series sharing one
+ * would otherwise resolve to the same delta.
+ */
+export function createDeltaLookup(
+  series: ChartSeries[]
+):
+  | ((seriesIndex: number, dataIndex: number) => ChartTooltipDelta | undefined)
+  | undefined {
+  const deltasBySeries = series.map((item) => item.deltas)
+
+  if (!deltasBySeries.some(Boolean)) return undefined
+
+  return (seriesIndex, dataIndex) =>
+    deltasBySeries[seriesIndex]?.[dataIndex] ?? undefined
+}
diff --git a/packages/charts/src/internal/series/tests/series.test.ts b/packages/charts/src/internal/series/tests/series.test.ts
new file mode 100644
index 0000000000..78c894564e
--- /dev/null
+++ b/packages/charts/src/internal/series/tests/series.test.ts
@@ -0,0 +1,115 @@
+import { describe, expect, test } from 'vitest'
+
+import type { ChartSeries } from '../series'
+import {
+  collapseSeries,
+  createDeltaLookup,
+  defaultMaxSeries,
+  seriesLimit,
+} from '../series'
+
+describe('collapseSeries', () => {
+  test('leaves the series untouched when they fit within maxSeries', () => {
+    const series: ChartSeries[] = [
+      { name: 'A', data: [1] },
+      { name: 'B', data: [2] },
+      { name: 'C', data: [3] },
+    ]
+
+    expect(collapseSeries(series, 'Others', 3)).toBe(series)
+  })
+
+  test('folds the tail into a single aggregate named from othersLabel', () => {
+    const result = collapseSeries(
+      [
+        { name: 'A', data: [1, 2] },
+        { name: 'B', data: [3, 4] },
+        { name: 'C', data: [5, 6] },
+        { name: 'D', data: [7, 8] },
+      ],
+      'Others',
+      3
+    )
+
+    expect(result.map((s) => s.name)).toEqual(['A', 'B', 'Others'])
+    expect(result[2]?.data).toEqual([12, 14])
+  })
+
+  test('clamps maxSeries to the palette limit', () => {
+    expect(seriesLimit).toBe(6)
+
+    const result = collapseSeries(
+      Array.from({ length: 10 }, (_, i) => ({ name: `S${i}`, data: [1] })),
+      'Others',
+      99
+    )
+
+    expect(result).toHaveLength(seriesLimit)
+    expect(result[seriesLimit - 1]?.name).toBe('Others')
+  })
+
+  test('keeps the aggregate null where every folded series is null', () => {
+    const result = collapseSeries(
+      [
+        { name: 'A', data: [1, 1, 1] },
+        { name: 'B', data: [1, 1, 1] },
+        { name: 'C', data: [null, 5, null] },
+        { name: 'D', data: [null, null, 7] },
+      ],
+      'Others',
+      defaultMaxSeries
+    )
+
+    expect(result[2]?.data).toEqual([null, 5, 7])
+  })
+})
+
+describe('createDeltaLookup', () => {
+  test('returns undefined when no series carries deltas', () => {
+    expect(
+      createDeltaLookup([
+        { name: 'A', data: [1] },
+        { name: 'B', data: [2] },
+      ])
+    ).toBeUndefined()
+  })
+
+  test('resolves a delta by series and category position', () => {
+    const lookup = createDeltaLookup([
+      {
+        name: 'A',
+        data: [10, 20],
+        deltas: [
+          { value: '12%', direction: 'up' },
+          { value: '4%', direction: 'down' },
+        ],
+      },
+      { name: 'B', data: [30] },
+    ])
+
+    expect(lookup?.(0, 0)).toEqual({ value: '12%', direction: 'up' })
+    expect(lookup?.(0, 1)).toEqual({ value: '4%', direction: 'down' })
+    // B has no deltas.
+    expect(lookup?.(1, 0)).toBeUndefined()
+    // Out-of-range positions resolve to undefined, not a thrown lookup.
+    expect(lookup?.(5, 5)).toBeUndefined()
+  })
+
+  test('keeps deltas apart for two series sharing a name', () => {
+    const lookup = createDeltaLookup([
+      {
+        name: 'Sales',
+        data: [10],
+        deltas: [{ value: 'first', direction: 'up' }],
+      },
+      {
+        name: 'Sales',
+        data: [20],
+        deltas: [{ value: 'second', direction: 'up' }],
+      },
+    ])
+
+    expect(lookup?.(0, 0)?.value).toBe('first')
+    expect(lookup?.(1, 0)?.value).toBe('second')
+  })
+})
diff --git a/packages/charts/src/internal/tooltip/tooltip-echarts.ts b/packages/charts/src/internal/tooltip/tooltip-echarts.ts
index 076fb757ee..a04d7811f8 100644
--- a/packages/charts/src/internal/tooltip/tooltip-echarts.ts
+++ b/packages/charts/src/internal/tooltip/tooltip-echarts.ts
@@ -8,7 +8,7 @@ import { renderChartTooltip } from './tooltip-render'
  * locally because the engine doesn't re-export that type from `echarts/core`
  * or `echarts/components`.
  */
-interface EChartsAxisTooltipParam {
+export interface EChartsAxisTooltipParam {
   name?: string
   seriesName?: string
   value: unknown
@@ -17,7 +17,7 @@ interface EChartsAxisTooltipParam {
   dataIndex?: unknown
 }
 
-interface EChartsTooltipPositionSize {
+export interface EChartsTooltipPositionSize {
   contentSize: [number, number]
   viewSize: [number, number]
 }