Feature/twinx

Support for multiple axes.

![screen shot 2014-09-03 at 11 06 44 am](https://cloud.githubusercontent.com/assets/1078448/4137131/533f729e-3384-11e4-8d3d-b61e8f2a742b.png)
Sorry, I don't follow this at all. There's a 1-to-many relationship between plots and ranges, and then for each range there is a 1-to-many relationship to renderers that might want to make use of that range. The thing that links them up is the name associated. What exactly are you proposing?