Handle charts with zero contributions

This fixes #3 by better handling completely empty charts. All empty cubes are now the minimum height and the stats properly display now.

![screen shot 2014-04-01 at 9 32 58 am](https://cloud.githubusercontent.com/assets/6104/2579537/858ba08a-b9a2-11e3-8b8c-40e54fde4b79.png)

This fix will be available in v0.8.2.