Hi
Siegfried,
To add the vertical line in the chart, we recommend using the Annotation
feature instead of the ChartCrossHairBehavior. The Annotation allows you to add
both vertical and horizontal lines to the chart, and you can dynamically set
their positions programmatically.
To assist you further, we have provided a user guide for
Horizontal and Vertical Line Annotation in our documentation, which you can
refer to for detailed instructions. The guide will walk you through the process
of adding and customizing vertical and horizontal lines in the chart.
UG Link: Horizontal and Vertical Line Annotation in WPF
SfChart.
Additionally, to define the X and Y values for the annotation
lines based on the axis, you can refer to the documentation section on setting
values for annotations.
UG Link: Setting Annotation value based on Axis
Sample output:

We hope this suggestion helps to address your requirement
effectively. If you have any further questions or need assistance with
implementing this solution, please feel free to reach out to us.
Regards,
Dhanaraj
Rajendran.