xaml translatetransform.X binding to value

HI,

  I'm relatively new to some of Xaml features.. 

I'm trying to implement this progressbar with an arrow on top that shows the progress indicator in addition to the filled bar. I used polygon to display the arrow and just need to translate it in X direction to display it on top of the progrssBar. However, the binding applied below ...

Binding PlaybackProgressTip

which is a double is causing the code to crash. Can someone please tell me how to bind the TranslateTransform.X to the double value or I need to do something else?

Thanks,

<Grid x:Name="grid" VerticalAlignment="Top" Margin="0,34,0,0" Grid.Row="1" Height="18" d:LayoutOverrides="VerticalAlignment, Width, VerticalMargin"><ProgressBar x:Name="Progress" Maximum="1" Value="{Binding PlaybackProgress}" VerticalAlignment="Center"  Background="LightGray" Height="6" Visibility="Collapsed"  IsIndeterminate="False" Foreground="DarkBlue"/>

..
..
<Polygon.RenderTransform>
<TransformGroup>
<TranslateTransform x:Name="ProgressPositionTranslate" X="{Binding ProgressBarTip}" /> 								<ScaleTransform x:Name ="xformScale" ScaleX="1" ScaleY="1" /></TransformGroup>
</Polygon.RenderTransform>
</Polygon></Grid>