Common plotting library for the Scanpy ecosystem

I was wondering if plotting could be facilitated and made more consistent across
the Scanpy ecosystem. I envisage a library ("scanpyplot" or whatever) that

• provides building blocks for single-cell-related plots which can be re-used across the ecosystem
• provides helper functions for handling colors, saving figures, etc.
• encourages a consistent plotting API (e.g. by defining abstract base classes)
  • See also: https://github.com/theislab/scanpy/issues/956

Motivation:

• Even within scanpy, there is quite some duplicated code in the plotting section. Reducing duplicated code reduces the maintenance burden in the long run. Across the ecosystem there's even more duplicated plotting code:
  • While implementing the scirpy.pl.clonotype_network function, I found myself copying over a lot of code from the scanpy.pl.paga and scanpy.pl.dotplot functions.
  • scvelo also copied most of the paga code
  • scvelo has its own scatter
• All the scanpy helper functions for plotting (e.g. savefig_or_show, _set_color_for_categorical_obs etc.) are private scanpy functions. Implementing plotting functions with consistent bahaviour requires either to duplicate a lot of code, or to rely on a potentially unstable API. In fact, e.g. scvelo has duplicates of most of these scanpy helper functions. Squidpy has similar functions, too.
• I know at least three ecosystem libraries that implement some variant of a stacked barplot based on AnnData.obs (dandelion.pl.stackedbarplot by @zktuong, sc_toolbox.api.plot.cluster_composition_stacked_barplot by @Zethson, and scirpy.pl.group_abundance by myself). Maybe such more general plots based on AnnData could be part of a central library as well.

Start by reviewing the plotting section around scanpy.pl.paga and scanpy.pl.dotplot, along with the referenced helpers savefig_or_show and _set_color_for_categorical_obs. Compare the duplicated plotting utilities described for scvelo, Squidpy, and other ecosystem libraries, then clarify the library’s scope and API before implementation; the issue does not define a concrete completion test.