fix area/line chart xaxis tickformat

Fix bug that xlabel shows incorrect value when xlabel is numeric

Before
![2014-12-24 11 51 49](https://cloud.githubusercontent.com/assets/8503346/5545544/1789fd54-8b69-11e4-8e9a-2b10f084a14b.png)

After
![2014-12-24 11 52 52](https://cloud.githubusercontent.com/assets/8503346/5545545/1a910de4-8b69-11e4-9d9d-22614b079690.png)