Some short lines added with plot() are missing in the PNG output depending on DPI

[tskir]

Bug report

Depending on DPI, a figure saved as PNG is missing some short lines.

Code for reproduction

import matplotlib
from matplotlib import pyplot as plt
import numpy as np

matplotlib.use('agg')

# Generate a set of 50 random points.
np.random.seed(0)
all_x, all_y = (np.random.rand(50) for _ in range(2))

# Scatter dots in blue.
plt.scatter(all_x, all_y, c='blue', s=2)

# Plot very short lines (length 0.001) in red with a small offset on Y.
for x, y in zip(all_x, all_y):
    plt.plot((x, x + 0.001), (y + 0.01, y + 0.01), c='red', linewidth=2)

# Save with different resolutions
plt.savefig('bug_96.png', dpi=96)
plt.savefig('bug_300.png', dpi=300)

Actual outcome

Depending on export DPI, some short segments (in red, added by plot()) are missing, while all dots (in blue, added by scatter() are always present).

At 96 DPI:

At 300 DPI:

Expected outcome

The segments should be always present, regardless of the resolution.

Matplotlib version

• Operating system: Ubuntu 20.04.2 LTS
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): 3.4.2
• Matplotlib backend (print(matplotlib.get_backend())): agg
• Python version: 3.8.5

Python is installed via apt-get, Matplotlib installed system-wide using PIP.

[tskir]

Interestingly enough, the bug disappears with matplotlib.use('cairo').

At 96 DPI with cairo:

At 300 DPI with cairo:

[jklymak]

If you turn snapping off this is fixed. plot(....snap=False)

[jklymak]

FWIW, I think snapping for ticks makes some sense. I don't think there are many situations where having snapping on for other artists is a good idea at all. I'm not familiar enough with matplotlib's history wrt snapping, but it seems a bad idea in general.

I also recommend strongly getting around any bad ideas matplotlib may have had by saving at relatively high dpi and then using a graphics package to reduce the size optically. In my opinion we try to do too many "smart" things with low resolution images that really should be done in post-processing, not while we compose the images.

[tskir]

@jklymak Indeed, thank you very much for the pointer and for such a quick reply!

I'll apply this fix, and will leave this issue up for any related discussions.