Hi Asish!
The z-index is determined by the order of elements in the visual tree, so your ellipse is behind your chart. Additionally, higher z-indexes are displayed above lower z-indexes so you have them the wrong way round.
I found this XAML to work:
<Grid MouseMove="Grid_MouseMove" Name="grid">
<TextBox Background="AliceBlue"/>
<Ellipse Name="ellipse" HorizontalAlignment="Left" VerticalAlignment="Top" Width="10" Height="10" Fill="Aqua"/>
</Grid>
C#:
private void Grid_MouseMove(object sender, MouseEventArgs e)
{
var pos = e.GetPosition(grid);
ellipse.Margin = new Thickness(pos.X, pos.Y, 0, 0);
}