I wonder, how to create this binding, since Line.X2 IS NOT dependency property! :(
<Line Y1="0" X1="0" Y2="0" Stroke="Blue" StrokeThickness="3" Margin="0 -2 0 -2" X2="{Binding Path=RenderSize.Width, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type StackPanel}}}"/>
Line.X2 is a dependency property so you can do this in the normal way: