Bug report

Bug summary

The axes of a subplot_mosaic show up in a random order in fig.axes (likely due to the use of a set for uniquification in _identify_keys_and_nested).

Code for reproduction

for _ in $(seq 10); do python -c 'from pylab import *; fig, axs = subplot_mosaic("ab"); print(fig.axes.index(axs["a"]))'; done

Actual outcome

1
0
1
1
1
0
0
0
0
1

Expected outcome

Axes should be added in a consistent order. I guess a reasonable one would be as if iterating the spec in C order (dropping duplicates).

Not release critical (especially as the order was not fixed, so fixing an order is not a backcompat break), but would be nice to get this sorted out before the API moves out of being experimental.

Matplotlib version

• Operating system: linux
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): head
• Matplotlib backend (print(matplotlib.get_backend())): any
• Python version: 39
• Jupyter version (if applicable):
• Other libraries:

Note: the simple solution of replacing unique_ids = set() by unique_ids = cbook._OrderedSet() is good enough for the non-nested case, but doesn't handle nested layouts such as subplot_mosaic([["a", [["b1", "b2"], ["b3", "b4"]]], ["c", "d"]]) because currently the nested submosaic is always added after all the non-nested axes.