private void GraphicsLayer_MouseLeftButtonDown(object sender, ESRI.ArcGIS.Client.GraphicMouseButtonEventArgs e)
{
// close any open MapTip
MapTip.IsOpen = false;
// show the map tip for the graphic
MapTip.DataContext = e.Graphic;
MapTip.Anchor = MyMap.ScreenToMap(e.GetPosition(LayoutRoot));
MapTip.IsOpen = true;
}