[category axis id, value axis id].
col: columns, categories along a horizontal axis. bar: horizontal bars, categories up a
vertical axis, the first at the bottom, and each cluster's first series lowest.
Every bar in the group, in px.
The gap between clusters as a percentage of a bar's width.
-100..100: the share of a bar's width neighbors overlap by. A painter's gap is its negation.
OptionalseriesStacked bars: a line from each series' end in a category to its end in the next.
A bar group. Stacked series are stacked in order, the first nearest the category axis.