Simple to solve problem, making issue just to refer to it in the PR.

Bug report

For lines, Axes.get_tightbbox relies on Line2D.get_window_extent, which computes extra padding due to markers as

ms = (self._markersize / 72.0 * self.figure.dpi) * 0.5

but this can cut off half of the marker edge.

This phenomena which is only visible if you have a line that extends outside of a plot, and has markeredgewidth > 1. For simple markers, (like 'S', 'O'), the code should instead read

extra_pts = self._markersize + self._markeredgewidth
ms = (extra_pts / 72.0 * self.figure.dpi) * 0.5

I'll edit this shortly to include code that reproduces the bug, after I document for all the common markers what the expected behavior should be.

Matplotlib version

• Operating system: Linux
• Matplotlib version: 3.2.0rc2.post1594.dev0+gead45c7e6
• Matplotlib backend: module://ipykernel.pylab.backend_inline`
• Python version: 3.7.3
  MPL installed from Github, Python is miniconda.