R1:f91bef64f163

Stack chart functions in a more physical way

Summary:
Ref T13279. See that task for some discussion.

The accumulations of some of the datasets may be negative (e.g., if more tasks are moved out of a project than into it) which can lead to negative area in the stacked chart.

Introduce `min(...)` and `max(...)` to separate a function into points above or below some line, then mangle the areas to pick the negative and positive regions apart so they at least have a plausible physical interpretation and none…