Adding a method to stack transitions in objects with several parts

[carlosvpi]

What

This PR adds a call named stackTransition that enables composite shapes (like a pie chart or a stacked bar in a bar chart) have a coherent transition.

This solution allows to use any easing function (d3 or custom), and the composite object will transition, as a whole, the same way a simple one would, giving a smooth feeling to the overall animation.

Stacking.transitions._.Carlos.Vazquez._.Observable.mp4

How

stackTransition sets the delay, duration and easeVarying of each component inside the composite element.
They all start the transition at the same time as the overall figure (same delay) and take the same time for the transition (same duration).

The ease of each element varies, however, in such a way that t becomes 0 before their time to start, and 1 when they finished. All elements have the easing parameter (t) bound to [0, 1], except the first, which is open on the left (can go below 0) and the last, which is open on the right (can go above 1), in order to allow for any easing that goes beyond 0 or 1 at any point (like elastic easing)

Example

https://observablehq.com/@carlos-vazquez/stacking-transitions