PR Summary

test_constrainedlayout.py::test_constrained_layout17 is a bit flakey because the constraints are poor. In general, constrained layout needs to be able to handle the situation below where the bottom axes is twice as large as the upper two, but the outer spines must be aligned.

However, it also has to work in the case where there are no inner spines to align:

This case was flakey before because I can only strong constrain the widths of the larger axes to be at least twice as large as the smaller. If there are also spines to line up, then this works fine, but in the second example above, it doesn't constrain the smaller axes from being any size smaller than twice the width of the bigger axes.

The change here is to introduce a 'weak' constraint that is ignored in the first case, and executed in the second case.

This is a follow up to #17306

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way