Bug report

Bug summary

Axes.plot produces thin lines (1, 3, ... px) which may snap incorrectly relative to Rectangles depending on the dpi and Axes position, in other words, the width of the left and bottom margins. In such cases, non-snapped lines achieve the desired outcome.

Code for reproduction

from matplotlib.pyplot import figure
from matplotlib.patches import Rectangle

fig = figure(figsize=(10,10), dpi=155)
ax = fig.add_axes([0.09, 0.1, 0.8, 0.86])
ax.axis([0, 1240, 0, 1333])
ax.minorticks_on()
ax.grid(which='both', ls=':')

ax.add_patch(Rectangle((700,800), 200, 3, snap=True))
ax.add_patch(Rectangle((800,700), 3, 200, snap=True))
lw = 1 * 72 / 155
ax.plot([890,801.5,801.5], [801.5,801.5,890], c='lime', lw=lw, snap=True)
ax.plot([710,801.5,801.5], [801.5,801.5,710], c='red', lw=lw, snap=False)

fig.savefig("snap.png")

The Axes relative position of (0.09, 0.1) is chosen on purpose so that the left margin is 0.09 * 10 in * 155 dpi = 139.5 px and the bottom one is 155 px. Also note that the Axis are scaled to be 1 pixel : 1 unit.

Actual outcome

Zoom in to view individual pixels...

Expected outcome

That the lime line snaps correctly in the middle of the blue Rectangles. Note how the horizontal non-snapped red line works as desired.

Feature request

To implement something equivalent to snapx and snapy following scalex and scaley trend. #15595 might change this trend though. This feature could be used to work around this issue, but I think the solutions should be independent. Will split this request into another issue if it's welcomed.

Matplotlib version

• Operating system: Windows 10
• Matplotlib version: 3.3.3
• Matplotlib backend: module://ipykernel.pylab.backend_inline
• Python version: 3.9.0

Python installed via official installer. Matplotlib installed via pip.