Bezier curve for drawing an area under code with dynamic wpf data mapping

Question

I have a program that creates a chart with dynamicdatadisplay. I need to fill in the indicated area under the curve (not the entire curve, just part, for example, if I have a sine from 0 to 1 on the x axis, only the area under the curve that corresponds to 0.2 to 0.5 in x axis) .

Does anyone know how to do this? I was thinking, maybe I could make a Bezier curve with Y values ​​that correspond to this segment of the sine curve, and fill it, but I don't get a command to draw something.

<d3:ViewportPolyBezierCurve x:Name="vpbc" BuildBezierPoints="True" BezierPoints="0.1,1,0.3,0.4,5,0.5" Fill="Red"></d3:ViewportPolyBezierCurve> 

Does anyone know how to make beziercurve with dynamic data mapping?

I could also implement the Bezier curve using the Path Geometry Figure, but then when I insert this into my plot and I do the scaling, the graph changes size, but the Bezier curve doesn't!

Any help would be really appreciated!