Stacked Area Graph (READY)

The beginning of what needs to happen to get stacked graphs working.

- [x] Add a `stack` data selection to derive new data in stacked format.
- [x] Refactor data collection/selection and graph so that derived data is available when needed.
- [x] Update the stacked-area.html example.
- [x] Add all related stacked-area config to graphBuilder for ease of use.