add chart intervals (bug 819123)

![](http://f.cl.ly/items/0V3y3A1Z170w2b0B1D3i/Image%202014-10-23%20at%202.47.07%20PM.png)
Yea all our chart types are a time-series at the moment and the API accepts an `interval`. If/when that changes it should be a conditional include.