Unfortunately the functionality you are seeking is not supported by RadChart. We have created a new charting solution that works better in DateTime scenarios. You can find more information and comparison between RadChart and RadChartView in our help system here.
I have created a small application that demonstrates how you can use DateTimeContinuous axis in your scenario.
Hope this helps!
All the best,
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>