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For some charts, you might want a free-floating marker with a crosshair design to emphasize a data point. In this topic, you'll build on the line and dot marker by adding a horizontal marker as well. The completed chart with marker will resemble the following image:

In the XAML below, you'll set the DataPoint to NaN again:
<!-- crosshairs -->
<c1:ChartPanelObject x:Name="vline"
Attach="None"
Action="MouseMove"
DataPoint="-1, NaN"
VerticalContentAlignment="Stretch"
HorizontalAlignment="Center">
<Border Background="Red" BorderBrush="Red" Padding="1" BorderThickness="1 0 0 0" />
</c1:ChartPanelObject>
<c1:ChartPanelObject x:Name="hline"
Attach="None"
Action="MouseMove"
DataPoint="NaN, -1"
HorizontalContentAlignment="Stretch"
VerticalAlignment="Center">
<Border Background="Red" BorderBrush="Red" Padding="1" BorderThickness="0 1 0 0" />
</c1:ChartPanelObject>
<c1:ChartPanelObject x:Name="dot"
Attach="None"
Action="MouseMove"
DataPoint="0.5,0.5"
HorizontalAlignment="Center"
VerticalAlignment="Center">
<Grid DataContext="{Binding RelativeSource={x:Static RelativeSource.Self},Path=Parent}">
<Ellipse Fill="White" Stroke="Red" StrokeThickness="1" Width="30" Height="30" />
<TextBlock x:Name="label" Text="{Binding DataPoint.Y, StringFormat=n0}" FontWeight="Bold" VerticalAlignment="Center" HorizontalAlignment="Center"/>
</Grid>
</c1:ChartPanelObject>