Questions

It’s a bit of a hack, but I’ve got a solution that works.

I used a custom SeriesSelectionModifier that could be instructed to ignore the next mouse up:

class CustomSeriesSelectionModifier : SeriesSelectionModifier
{
    public bool IgnoreNext { get; set; }

    public override void OnModifierMouseUp(ModifierMouseArgs e)
    {
        if (IgnoreNext == false)
        {
            IgnoreNext = false;
            base.OnModifierMouseUp(e);
        }
        IgnoreNext = false;
    }
}

and hooked that up to the DragEnded event on the VerticalLineAnnotation:

line.DragEnded += (sender, args) =>
{
    customSeriesSelectionModifier.IgnoreNext = true;
};

Of course I also had to use the custom ChartModifier in the xaml:

<s:SciChartSurface.ChartModifier>
    <s:ModifierGroup>
        <s:LegendModifier x:Name="LegendModifier" />
        <s:ZoomPanModifier ExecuteOn="MouseRightButton" ClipModeX="None" />
        <s:RubberBandXyZoomModifier ExecuteOn="MouseLeftButton" />
        <customChartModifiers:CustomSeriesSelectionModifier />
        <s:ZoomExtentsModifier ExecuteOn="MouseDoubleClick" />
        <s:MouseWheelZoomModifier />
    </s:ModifierGroup>
</s:SciChartSurface.ChartModifier>