A delegate method for the filling is a good idea. 

But this creates a new question: When filled and non filled lines are combined, e.g. the lower line is non filled and the higher one is, should only the space between the higher and the lower be filled or everything between the higher line and the x-axis?
I would prefere the latter since otherwise it would look a bit strange.

But I would keep the property for the cumulative lines as this can be used intependently from the filling and it affects all lines.