Bug report

Bug summary

inset_locator.mark_inset() sometimes misplaces the box connector lines if x/y limits on the parent Axes have not been explicitly set. Updating the plot (e.g. by saving the figure) seems to fix it.

Code for reproduction

import matplotlib.pyplot as plt
import mpl_toolkits.axes_grid1.inset_locator as miloc

fig, (inset, full) = plt.subplots(ncols=2)

# swapping the axes makes the example work
# full, inset = inset, full

full.plot([-1, 0], [-1, 0])
inset.set_xlim(-0.6, -0.4)
inset.set_ylim(-0.6, -0.4)

# workaround
# full.set_xlim(-1.0, 0.0)
# full.set_ylim(-1.0, 0.0)

miloc.mark_inset(full, inset, 1, 4)
fig.savefig('Bug.png')
# saving the figure a second time produces the expected result
fig.savefig('NoBug.png')

Actual outcome

Expected outcome

This issue was not present in version 3.0.2.

Matplotlib version

• Operating system: Fedora and Debian
• Matplotlib version: 3.2.1 and 3.2.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg and agg
• Python version: 3.8
• Jupyter version (if applicable): N/A
• Other libraries: N/A

Installed via pip on Debian and both pip and dnf on Fedora.