[refactor] Add typing to Plots

Note, this is targetted at api-breaking-changes, not develop.
Actually, it is in scope, and this probably hints why it is an antipattern.  In the documentation pr, these yScales are underscored for protection, but there's no way to predict this from this pr's standpoint.  Because this pr can't detect this, it will be completely out of sync with the right yScale