I already answered your question in the support ticket that you opened. I will post the answer also here so that it will be available for the other members of our community.
The RadChartView doesn't support an annotation with handlers out of the box. However, you should be able to achieve your requirement with custom implementation. For example you can have two CartesianGridLineAnnotations which will represent the handlers. The annotations can be placed at the same start and end dates of the CartesianMarkedZoneAnnotation. Then you can subscribe (for example) for the MouseMove event of the chart view and if the mouse is under the handler's annotation, and based on the mouse position, you can move the annotations. You can use the ConversionAPI to get the data point under the mouse and use its date.
Another approach is instead of the MouseMove to use drag/drop events. I prepared a sample project with the DragDropManager that demonstrates resizable marked zone annotation. Please keep in mind this is a custom implementation that is not well tested and I cannot guarantee that it will work correctly in all possible scenarios. This is why I suggest you to use the approach from the project only as a base for your implementation
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