How to fix VerticalLineAnnotation always setting binding to zero

I'm trying to set up a SciChart VerticalLineAnnotation similar to the Drag Horizontal Threshold example in the SciChart example suite. I have the annotation bound to a property in my view model, but when dragging the annotation it continually sets the property to a value of 0.

I've tried setting the property value in code and it does move the line as you would expect indicating the binding is fine. I set a breakpoint on the property and confirmed the 0 value is coming from the control, not my code.

<sc:SciChartSurface x:Name="OverviewSurface"
                    Margin="5,2,5,2"
                    Background="White"
                    Grid.Row="2"
                    Grid.ColumnSpan="4"
                    Loaded="OnOverviewSurfaceLoaded"
                    RenderableSeries="{Binding ElementName=ChartSurface, Path=RenderableSeries}"
                    YAxes="{sc:AxesBinding YAxes}">
    <sc:SciChartSurface.XAxis>
        <sc:NumericAxis VisibleRange="{Binding VisibleRange}" DrawMajorGridLines="False" DrawMinorGridLines="False" DrawMajorBands="False" Visibility="Collapsed"/>
    </sc:SciChartSurface.XAxis>
    <sc:SciChartSurface.Annotations>
        <sc:VerticalLineAnnotation VerticalAlignment="Stretch"
                                    IsEditable="True"
                                    ShowLabel="False"
                                    Stroke="#FF42b649"
                                    StrokeThickness="4"
                                    YAxisId="ChannelA"
                                    X1="{Binding SelectedIndex, Mode=TwoWay}" />
    </sc:SciChartSurface.Annotations>
</sc:SciChartSurface>

I expect that when I drag the Annotation along the horizontal length of the chart that I will get a value corresponding with the X position placed in SelectedIndex, but all I ever get is zero.

Solution

The cause of this issue turned out to be the data type of SelectedIndex, which was an unsigned integer. Despite the fact that VerticalLineAnnotation's X/Y value type is IComparable, if you are using the IsEditable="True" function, the drag step size is fractional and if bound to an integer type, it will continually round to 0 making it incapable of movement.

If there is a way to change the step size of the drag in SciChart, I was unable to locate it. Instead I changed SelectedIndex to a double and rounded it to my desired unsigned integer in the setter, which solved my problem.