add ratio to chart

In our project we needed a graph with grids forming a square. 
The difficulty lies in the fact that our project is responsive. When changing the size of the window width of the graph changes but not the height. 
I propose to change the height of a graph using a ratio that the Directive receives. 
Thus the graph keeps fixed proportions user.

Here an example:
![ex](https://cloud.githubusercontent.com/assets/6442236/3443278/8ee38f92-0120-11e4-8a81-3dfb40623352.png)