The likely cause is a dictionary somewhere that causes the drawing order to be non-deterministic.

I found these in #5307, but decided to punt on resolving them for now -- instead just increasing the tolerance on those tests. This is a reminder that we may want to go back and investigate this at a later date.

The tests are:

• test_axes_grid1:test_twin_axes_empty_and_removed
• test_axes:test_boxplot
• test_axes:test_boxplot_rc_parameters
• test_axes:test_specgram_freqs
• test_axes:test_specgram_noise
• test_axes:test_specgram_magnitude_freqs