How to draw a horizontal line without specifying X2??

I want to draw a horizontal line that fills up the entire available width of
a grid cell. I don't want to explicitly specify the line's width (X2). How
do I do this???

<Line Grid.Row="1" Stroke="Gray" StrokeThickness="1" X1="0" X2="200" Y1="1"
Y2="1" />

in above sample, I want to remove the "200" so that the line sizes
automatically when the grid is resized. thanks!