Area graph & Stacked area graph (ready)

Just a first stab at this. Feedback appreciated.

- [x] A little example css cleanup.
- [x] Area component.
- [x] Area component example.
- [x] Factor in Y-baselines/Y-offsets when calculating domain in `graphs.graph`, thoughts on this?
- [x] Stacked area example.
- [ ] Need to encapsulate the stacked area code in example into the data stuff that @jverghese is doing.
  (deferring this until Johan's branch is merged)
- [ ] Would be nice to have a way to specify a default accessor function for all series in the data set.
  (deferring this)
- [x] Need clarification from UX on whether or not to have darker stroke above each area.  
  Resolved: will not show outline line for now.
- [x] Needs tests