[Repo Assist] fix(gcm/falsify): guard matplotlib import — prevents ModuleNotFoundError when importing falsify without [plotting]

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🤖 This is an automated PR from Repo Assist, an AI assistant for this repository.

Problem

gcm/falsify.py had two module-level matplotlib imports:

import matplotlib.colors as mcolors
import matplotlib.pyplot as plt

and a module-level constant that evaluated them immediately:

COLORS = list(mcolors.TABLEAU_COLORS.values())

This meant that merely importing dowhy.gcm.falsify (or any code path that loaded the module) would fail with ModuleNotFoundError: No module named 'matplotlib' in environments without the [plotting] extra, even when the caller had no intention of plotting anything.

plot_evaluation_results() is the only function in falsify.py that uses matplotlib directly. plot_local_insights() calls plot() from dowhy.gcm.util, which is already guarded by the companion PR #[task4-pr].

Fix

• Remove the two top-level matplotlib imports
• Remove COLORS = list(mcolors.TABLEAU_COLORS.values()) from module scope
• Add a lazy try/except guard + local colors list at the start of plot_evaluation_results():

try:
    import matplotlib.colors as mcolors
    import matplotlib.pyplot as plt
except ImportError:
    raise ImportError(
        "matplotlib is required for plotting. Install it with: pip install 'dowhy[plotting]'"
    )

colors = list(mcolors.TABLEAU_COLORS.values())

All COLORS[...] references inside the function are updated to colors[...].

VIOLATION_COLOR = "red" is a plain string constant and is unaffected.

Result

• import dowhy.gcm.falsify no longer requires matplotlib
• Calling plot_evaluation_results() without matplotlib gives a clear, actionable error message instead of a confusing ModuleNotFoundError

Test Status

Only import restructuring — no logic changed.
✅ flake8 --select=E9,F63,F7,F82 passes (0 hard errors).

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