V4 drawpolygon problem?

[Terry Fogg]

I am using
dc.DrawPolygon(new SolidColorBrush(Color.White), MyYellowPen, PolygonPoints)]
in a graph control I have created inherited from a panel.
The window has a stackpanel at the top with some text and a stack panel
below with this graph panel.
The polygon outline and filled parts do not line up, the polygon filled part
is offset vertically from the pen outline.
I can use a null pen to omit the display of the pen, but I can't use a null
brush to omit the fill without error.

I made the upper stackpanel larger by increasing the text it contained and
the offset increased.
It seems that the outline pen of the polygon is relative to the panel while
the polygon fill is relative to the top of the screen

Thanks Terry