Remove x axis padding on area chart

Great work on the module. Just providing a small change to prevent area charts from having padding on the x axis.

From this:
![screen shot 2014-06-21 at 17 07 44](https://cloud.githubusercontent.com/assets/2844580/3349355/5e59e99e-f95e-11e3-97fb-29042df45336.png)

To this:
![screen shot 2014-06-21 at 17 08 01](https://cloud.githubusercontent.com/assets/2844580/3349357/64d6f12c-f95e-11e3-9491-6efb6e7fbda8.png)