Bug summary

When the parameter X or Y is a masked array with a read-only mask, pcolor fails with ValueError: array is read-only

Code for reproduction

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 1, 10)
y = np.linspace(0, 1, 10)
X, Y = np.meshgrid(x, y)
Z = np.sin(2 * np.pi * X) * np.cos(2 * np.pi * Y)

mask = np.broadcast_to([True, False] * 5, Z.shape)
masked_X = np.ma.array(X, mask=mask)
masked_Y = np.ma.array(Y, mask=mask)
masked_Z = np.ma.array(Z, mask=mask)

plt.pcolormesh(masked_X, masked_Y, masked_Z)

Actual outcome

Traceback (most recent call last):
File "", line 1, in
File "/Library/Python/3.9/lib/python/site-packages/matplotlib/pyplot.py", line 2773, in pcolormesh
__ret = gca().pcolormesh(
File "/Library/Python/3.9/lib/python/site-packages/matplotlib/init.py", line 1442, in inner
return func(ax, *map(sanitize_sequence, args), **kwargs)
File "/Library/Python/3.9/lib/python/site-packages/matplotlib/axes/_axes.py", line 6220, in pcolormesh
X, Y, C, shading = self._pcolorargs('pcolormesh', *args,
File "/Library/Python/3.9/lib/python/site-packages/matplotlib/axes/_axes.py", line 5713, in _pcolorargs
X, Y = [cbook.safe_masked_invalid(a) for a in [X, Y]]
File "/Library/Python/3.9/lib/python/site-packages/matplotlib/axes/_axes.py", line 5713, in
X, Y = [cbook.safe_masked_invalid(a) for a in [X, Y]]
File "/Library/Python/3.9/lib/python/site-packages/matplotlib/cbook/init.py", line 715, in safe_masked_invalid
xm = np.ma.masked_invalid(x, copy=False)
File "/Library/Python/3.9/lib/python/site-packages/numpy/ma/core.py", line 2360, in masked_invalid
res = masked_where(~(np.isfinite(a)), a, copy=copy)
File "/Library/Python/3.9/lib/python/site-packages/numpy/ma/core.py", line 1942, in masked_where
result.mask = _shrink_mask(cond)
File "/Library/Python/3.9/lib/python/site-packages/numpy/ma/core.py", line 3516, in mask
self.setmask(value)
File "/Library/Python/3.9/lib/python/site-packages/numpy/ma/core.py", line 3462, in setmask
current_mask.flat = mask
ValueError: array is read-only

Expected outcome

No error

Additional information

The error still exists because I missed the following code in fixing #26093:

And I will fix it as well if allowed.

Operating system

OS/X

Matplotlib Version

3.7.1

Matplotlib Backend

MacOSX

Python version

3.9

Jupyter version

No response

Installation

None