CLN: plotting cleanups for groupby plotting
Related to #8018. Because it touches lots of codes, I've split some cleanups required for #8018 which does NOT changes current functionality.

@sinhrks status?
 @TomAugspurger Can you take a look? It's nice if #8018 can be included in v0.17.
 this lgtm. so when you guys are ok, pls merge.
 Let me merge this if there is no objections in this weekend.
