(W.I.P) Area chart

[jamesrswift]

An in-progress implementation for area charts. I'm not happy with the shape of the API (trying to figure out whether people will want to group points by series or by x-coordinate).

I'm also hoping this this might help shape a general chart-like solution that can be reused to reduce boilerplate code.

To do:

• [ ] Marks and mark styles
• [ ] There's some weird anti-aliasing, I think I'm overdrawing
• [ ] Stacked100 area chart does the scaling calculation wrong
• [ ] Consider the shape of data people are likely to want to input. This is another argument for a generalized "data-shaper" thing
• [ ] Simple (not implemented yet)
• [ ] Overlayed (not implemented yet)
• [ ] Stacked: Started
• [ ] Stacked100: Started