Correct x axis interval

This code shows the correct intervals on x-Axis if the labels are dates.
Instead of this:
![normal_line](https://f.cloud.github.com/assets/4931746/752772/efa88e7c-e551-11e2-9bfa-238c0411f486.png)

->
![interval_line](https://f.cloud.github.com/assets/4931746/752771/efa386b6-e551-11e2-9664-936309112ee3.png)