fix line chart x axis minimum to 0, not min(y-data)

This PR will do this 

Before
![screen shot 2014-11-21 at 10 56 47 am](https://cloud.githubusercontent.com/assets/5210115/5136543/29ba6e52-716d-11e4-8637-5f554679ac28.png)

After
![screen shot 2014-11-21 at 10 56 24 am](https://cloud.githubusercontent.com/assets/5210115/5136545/2b51596a-716d-11e4-93e2-f4639d2e73a7.png)