I noticed PlottableBar implements an (effectively) unused, unexposed baseline (currently a single float). I want to plot stacked bars (e.g. bar 1 is from 0 to 2 and bar 2 is from 2 to 5). It is possible to get acceptable behavior for doing stacked bars in many cases with the current interface, however not in all (and in particular not in mine). Would you consider exposing an optional argument which allows one to specify a baseline array for the bars? This could also of course be useful for other things.