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)
the problem with setting the objects directly is that you have to remember to set it everywhere.  On the plot, on the extra axis, on all the renderers that want to utilize it. You have to carry the actual object around to all parts of your program that might build up different parts of your plot. This way you just have to set it on the plot with a name, and then use it more simply everywhere else.