PR Summary

This PR rewritesconstrained_layout. There are a couple of motivations. CL largely worked, but was prone to failure. Basically the model was arrange a bunch of axes boxes and have them with a certain minimum margin around themselves, and the axes in the same rows would have their spines aligned. The problem with that approach was that there was no constraint against all the axes being zero width and height, nor that they filled the figure. To get around that there was a weak constraint asking for all the axes to fill the figure, and then the solver would try and make them large rather than small.

However, this failed frequently. One of the tests was marked "flakey" because it would still occasionally collapse to zero width/height. This is obviously not robust nor particularly elegant.

The new approach is API equivalent, but much more robust. The only editable feature are the margins around the axes (there happen to be two of them, one for the decorations and the other for the padding and colorbars). These default to zero width, so the solution is always valid. Of course its still possible for the axes to be zero sized if the decorations are too large, in which case a warning is emitted.

Visually there are some substantial differences. The meaning ofwspace andhsapce was twice as wide as it should have been, at least according to the documentation, so that has been fixed.

More importantly, colorbars have been substantially improved. They now honour theanchor kwarg forcolorbar (closes#14638). This leads to wider colorbars as well, but I'd argue they are more "correct" than they used to be.

In terms of simplicity, the new version now only attaches itself to figures and gridspecs. There are no boxes attached to individual axes etc. Rather each gridspec is divided into columns and rows and all the layout information is contained at the gridspec level. There is still substantially messy bits to do with placing colorbars, largely because left, right, bottom and top placements are all different.

In terms of performance, 1) it actually improves non-CL performance somewhat, and CL performance as well. New version: Constrained: 14.31 s Not: 6.88 s. Old version: Constrained: 15.40 s Not: 7.30 s. So, the new version is actually 6% faster with CL turned off, and about 10% faster with it turned on.

Future advantages: This was motivated by implementing supxlabel and supylabel (#11147). the new layout allows gridspec-wide margins and thus suptitles, colorbars etc.

This will also allow subfigures/subpanels to be implemented recursively (there are stubs in here for this).

PR Checklist

• Has Pytest style unit tests
• Code isFlake 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