More rank plot color schemes

This could involve adding more options (i.e. more sets of 3 colors in the Color scheme box below the rank plot), or it could involve adding separate color selectors for the numerator / denominator / both fields, where each of those three selectors is hooked up to some sort of color picker.

The former idea is easier to do in the short term; the latter idea would be easier for users but would be much harder to implement (although not really that bad to implement, main thing as with #225 would just be adding tests).

Idea c/o @lisa55asil.