Adding horizontal plotlines to LineChart

HI, 

After looking at the source code I realized there was no easy way to add plotlines to a graph (it really makes sense to have these only on Lines and Bars).

So far I have only coded it for the Line.

Feel free to let me know what you think of it (implementation) and how to potentially port this for BarChart as well (may be these two charts should extends a "plotable lines" Chart?).

I hope you will like it !