wpf canvas resize

Dear all

I am slowly learning how to program wpf. One thing I am trying to do is plot a series of polylines on a canvas. I have gotton this far. However I would like the polylines to scale as I resize the canvas. I have a width property in my IValueConverter derived class, however I am struggling to get this to update the polylines on the resize event of the canvas. Any ideas what I am doing wrong ?

any help would be appreciated

simon