Consistent aesthetics

[ralsei]

Currently, only some of the renderers (points, lines, density) support aesthetics like #:discrete-color, only points supports #:continuous-color, et cetera. While setting certain aesthetics to some renderers does not make sense, it should be possible to abstract over this in a meaningful way.

See Figure 5.9 from socviz: which is currently not possible as there is no coloring aesthetic for boxplot.

I think this signals a need to clarify and understand what an "aesthetic mapping" is: mapping some feature of the plot to a variable in the data-set. Perhaps there should be a better correspondence between the keyword arguments and the aesthetics.