follow those steps:
1- add a line serie ( the delegate methode will now return two)
2- look at the demo and choose one function from there and then make it y = cstValue (where cstValue is a constant value where you want you horizontal straight to be drawn)
3- you have a two properties in the lineserie: linewidth for the width and lineColor. you know what are there for.
if you need code let me know