I have a graph with a polyline contained into a canvas. I would like to set the stroke of the polyline starting from a color at the bottom and ending to another color on top. I tried with this xaml:

<Polyline StrokeThickness="2">
  <Polyline.Stroke>
    <LinearGradientBrush StartPoint="0.5,1" EndPoint="0.5,0">
      <GradientStop Color="Blue"  Offset="0" />
      <GradientStop Color="Cyan" Offset="1" />
    </LinearGradientBrush>
  </Polyline.Stroke>
</Polyline>

In this way it works but the height of the gradient is equal to the height of the polyline. I mean, if I have a polyline which goes from the bottom of the canvas to its top, the gradient is applied over the entire height of the canvas. If I have instead a horizontal polyline, the gradient is applied to its thickness.
I would like to have a gradient height equal to the canvas height, independently from the polyline height.
How can I achieve this?