Better information scent about nesting

The current UI around nested charts is not bad in terms of information scent, it's totally on par with any good tree view.
But it _can_ do better.

Currently, until you expand, the only indication of the shape of the children is the number of children. However, both this:

and this:

have 3 children, but vastly different "shapes".

What if we could show this on the bar? Possibly not always if too noisy, but maybe just when the number of children is hovered.

Then, we could go even further and make the animation actually _communicate something_ rather than being mostly decorative, by making the bars unfold down from their places in the parent bar chart!