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)
I'm asking, because we have a (reasonably) well defined mechanism for expressing relations between objects and this is something new that compromises validation.