Bug report

Markers appear smeared and not clear

• When I plot markers with pyplot they show compression artifacts even when I save them as PNG.

Code for reproduction

#!/usr/bin/env python                                                          

import numpy as np
import matplotlib.pyplot as plt

x = [1, 2, 3]

plt.errorbar(x, x, xerr=.05, yerr=.02, fmt='+')

plt.savefig('test.png')

Actual outcome

See image attached.

Expected outcome

I want markers without smearing or interpolation as clear as the resolution permits.

Matplotlib version

• Matplotlib version 2.0.0, Python 2.7.13 and Debian 9.
• Matplotlib installed with pip

Why do you think these are compression artifacts?

What else?

I just realize now, that the asymmetry of the marker is caused by the fmt='+'. If I replace this by linestyle='', then the smearing becomes symmetric. I think then it is displayed correctly.
I am still surprised by the asymmetry when using fmt='+' but since it can be avoided it doesn't seem critical.

fmt='+' means each value gets a plus marker, then the errorbars are drawn on top. If you want no markers, then use fmt='none', but there's currently a bug there... #8163

This is a duplicate of #3400 due to fractional pixel offests between the lines used for the error bars and the marker.