Display values in Line charts

Hi there,

I've tried to add a display values option into Line charts functionality for a project, and it seems to work well.

Find below a (simple) example of use :

![capture decran 2013-07-05 a 16 15 16](https://f.cloud.github.com/assets/1038282/754103/59d87f92-e57e-11e2-815f-305f09927f23.png)

Feel free to test and review it, hope it will help some developers ;)