Graphing fills

Don't review until https://github.com/dimagi/commcare-odk/pull/106 is merged.

Fills aren't always drawn correctly when they cross canvas boundaries. For one example, this is fine...

![bug1](https://cloud.githubusercontent.com/assets/1486591/5559033/e29070f8-8d0a-11e4-9878-0d3a657e71f8.png)

...but upon zooming & panning, the fill is no longer drawn correctly: 

![bug2](https://cloud.githubusercontent.com/assets/1486591/5559036/f6cde10e-8d0a-11e4-99f9-14dcc6e75e83.png)