axes_grid1 has a bunch of great tools, but the API is sometimes idiosyncratic, its not well maintained, partially because the code is quite complicated. The best methods can and should be moved to the core library.
In general we're migrating to pytest for test discovery and execution.
The PRs should be merged into master. No backporting is necessary.
Better support for categorical (ordinal and nominal) data. Continuation of GSOC project, more information can be found at https://story645.github.io/
Documentation in transition to numpydoc and sphinx-gallery