Support explicit stacking configuration in stacked area charts
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.
Maniphest Tasks: T13279
Differential Revision: https://secure.phabricator.com/D20818