As a user, I want to have a responsive chart legend, like so:

Users can implement this manually today, via ChartLegend's itemsPerRow property, as shown by the bullet chart example above. However, I believe we my be able to do some of the work automatically. That is, providing the legend is generated via Chart.

Some investigation is needed, but Chart may be able to determine the legend's width, since it already has the SVG's padding. Then, we could set ChartLegend's itemsPerRow property if the legend is wider than the SVG, minus padding.

Note that this should be an opt-in feature, so we don't break existing user layouts.