WPF DataGrid 行头小三角

<DataTemplate x:Key="RowHeaderTemplate">
<StackPanel Orientation="Horizontal">
<Image Margin="" Height="">
<Image.Source>
<DrawingImage>
<DrawingImage.Drawing>
<GeometryDrawing Geometry="M 0 0 L 200 150 0 300 Z" Brush="LimeGreen">
<GeometryDrawing.Pen>
<Pen Brush="LimeGreen" Thickness="" />
</GeometryDrawing.Pen>
</GeometryDrawing>
</DrawingImage.Drawing>
</DrawingImage>
</Image.Source>
</Image>
<TextBlock Text="Source" VerticalAlignment="Center"></TextBlock>
</StackPanel>
</DataTemplate>

<toolkit:DataGrid x:Name="dataGrid1" Grid.Row=""
AutoGenerateColumns="True"
Background="Transparent"
SelectionUnit="CellOrRowHeader"
RowHeaderWidth=""
RowHeaderTemplate="{StaticResource RowHeaderTemplate}">