Support explicit stacking configuration in stacked area charts

Summary:
Ref T13279. Allow engines to choose how areas in a stacked area chart stack on top of one another.

This could also be accomplished by using multiple stacked area datasets, but datasets would still need to know if they're stacking "up" or "down" so it's probably about the same at the end of the day.

Test Plan:

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20818