Allow woking on different Graphs

Hej Alistair,
this PR will allow the user to switch between different graphs.

I'm not sure about the d3-binding for the combo-box,  please have a look if there is a more elegant way. 

cheers, Thomas

![image](https://f.cloud.github.com/assets/91751/489156/a2f8bde4-b992-11e2-8377-48e59dc66f5c.png)