R1:81456db5594e

Roughly support stacked area charts

Summary:
Ref T13279. This adds support for:

- Datasets can have types, like "stacked area".
- Datasets can have multiple functions.
- Charts can store dataset types and datasets with multiple functions.
- Adds a "stacked area" dataset.
- Makes D3 actually draw a stacked area chart.

Lots of rough edges here still, but the result looks slightly more like it's supposed to look.

D3 can do some of this logic itself, like adding up the area stacks on top of one…