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Fix: Prevent clabel() from modifying filled contour geometry#31348

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shivamrajsr07 wants to merge 1 commit intomatplotlib:mainfrom
shivamrajsr07:fix-contour-label-bug
Closed

Fix: Prevent clabel() from modifying filled contour geometry#31348
shivamrajsr07 wants to merge 1 commit intomatplotlib:mainfrom
shivamrajsr07:fix-contour-label-bug

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@shivamrajsr07
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@shivamrajsr07 shivamrajsr07 commented Mar 22, 2026

Summary

Fixes an issue where adding contour labels using clabel() on filled contour plots (contourf) alters the geometry of the filled regions.

When inline labeling is enabled, the labeling logic modifies internal path data. This behavior is appropriate for line contours but unintentionally affects filled contours, resulting in distorted shapes.

Root Cause

The inline labeling code updates the internal _paths of the contour set regardless of whether the contour is filled or not. For filled contours, these paths define the filled regions themselves, so modifying them changes the visual output.

Fix

Restrict path modification to non-filled contours only. Inline labeling now avoids mutating _paths when self.filled is True.

Reproduction

Example demonstrating the issue:

import matplotlib.pyplot as plt
import numpy as np

delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)

Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = (Z1 - Z2) * 2

fig, axs = plt.subplots(2, 1)

axs[0].contourf(X, Y, Z)

CS = axs[1].contourf(X, Y, Z)
axs[1].clabel(CS)

plt.show()

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@rcomer
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rcomer commented Mar 22, 2026 •
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Skipping the path breaking is indeed simple. However, the problem plot also wrongly shows two numbers on each boundary, and that is much harder to fix. As noted already in the issue comments and the existing PR, this was never expected to work for filled contours.

So I’m sorry but I’m going to close this.

@rcomer rcomer closed this Mar 22, 2026
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