Binding to Canvas.Left breaks when element is moving

I have the following Canvas which has 3 elements:

            <Rectangle x:Name="start" Canvas.Left="20" Canvas.Top="100" Width="25" Height="25" Fill="Pink"/>


            <Line X1="{Binding ConverterParameter=X, ElementName=start, Converter={StaticResource CanvasPositionConverter}}" 
                  Y1="{Binding ConverterParameter=Y, ElementName=start, Converter={StaticResource CanvasPositionConverter}}" 
                  X2="{Binding ConverterParameter=X, ElementName=root, Converter={StaticResource CanvasPositionConverter}}" 
                  Y2="{Binding ConverterParameter=Y, ElementName=root, Converter={StaticResource CanvasPositionConverter}}"  
                  Stroke="Red" StrokeThickness="2" />

          <!--I have something external that controls the position of this dockpanel on the canvas so on runtime it may move-->
            <DockPanel x:Name="root" Canvas.Left="0" Canvas.Top="0" Width="20" Height="20" />

where CanvasPositionConverter is this:

   public object Convert(object value, Type targetType, object parameter,
        System.Globalization.CultureInfo culture)
    {

        if (parameter.ToString().ToLower() == "x")

            return Canvas.GetLeft(value as FrameworkElement);

        else if (parameter.ToString().ToLower() == "y")

            return Canvas.GetTop(value as FrameworkElement);


        else throw new Exception("Need a ConverterParameter of either X or Y");
    }

It does work on startup, meaning the initial values of the bindings are correct (as shown in Snoop); the problem is the dockpanel is moving at runtime but the bindings do not update to the new location of the dockpanel. Basically I want a line that connects a static shape to a moving shape. (The moving shape is actually in an adorner which the user can drag to move)