One way of solving this is using Range series for example. You could also use Stack100 CombineMode of your series to stack two area series. Filling the graph below, can be achieved by manually specifying a range of your axis
. For that purpose use the Minimum, Maximum and MajorStep properties of your vertical linear axis.
In case this is not what you expected, could you please elaborate a bit more on your scenario so we could give you a better solution.