Bug summary

Pixel widths appear irregularly in pcolor(x, y, z, rasterized=True), while x and y are regular. Using rasterized=False provides correct (equal size) pixels, but saving the figure to .png (instead of .pdf) will show the same irregular pixel width bug.

It looks like a resampling bug, which messes up precise data visualization completely.

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt
x, y = np.meshgrid(range(500), range(2))
z = [np.tile([0,1],250), np.tile([0,1],250)]
fig, ax = plt.subplots(1,1, figsize=(10,2))
im = ax.pcolor(x, y, z, rasterized=True)
ax.set_xticks(np.arange(0,40))
fig.savefig('test.pdf')
plt.show()

Actual outcome

In the resulting plot, some pixels are thinner, and some are thicker, than they should be.
rasterize=True (zoom-in to the lower left corner)

Expected outcome

rasterize=False (zoom-in to the lower left corner)

Additional information

• This behaviour depends on the number of pixels plotted, for me it becomes visible above len(x)>100.
• The spread between thicker and thinner lines increases with increasing size of the input array.
• If the view is reduced to a lower number of pixels (e.g., ax.set_xlim(0,10)), the effect is no longer visible.

Operating system

Windows

Matplotlib Version

3.5.1

Matplotlib Backend

inline

Python version

3.9.6

Jupyter version

No response

Installation

pip