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 of the areas are negative.

This is presumably not a final version, I'm just trying to produce a chart that isn't a sequence of overlapping regions with negative areas that is "technically" correct but not really possible to interpret.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20506