Bug report

Bug summary

Marker edge overlaps with marker and the edge appears to be double the width specified. This becomes obvious when you use alpha transparency and a very larger point marker with a large edge width or in the case where I noticed it a small figure (about 2 inches wide) with the default rcParams.

Code for reproduction

Case 1: very large point with a large edge width

plt.figure()
plt.plot(1, '.', ms=200, mew=50, alpha=0.5)

Case 2: very small figure with default rcParams: markersize = 8, markeredgewidth=1.0

plt.figure(figsize=(1,1), dpi=600)
plt.plot(1, '.', alpha=0.5)

Actual outcome
In this figure I have markersize=200 and markeredgewith=50. There is an ugly ring and it looks as if the marker edges are two times wider (100 instead of 50) than I asked.

Now in this figure I have markersize=200 and markeredgewidth=99. There is a single pixel in the center that is faded blue thus the markeredges are definitely twice what I asked for. If I set the markeredgewith to 100 I get a solid core.

Expected outcome
I would expect that the marker edge would line up perfectly with the marker core. In this plot I set markersize to 300 (200 plus 2*50 edges) to get the same size as the original without the ugly ring.

Matplotlib version

• Operating system: macOS 10.13
• Matplotlib version: 3.0.2
• Matplotlib backend: both with Qt and PGF backends
• Python version: 3.7.2
• matplotlib installed via pip