Hi Frans van Ek,
I am afraid this functionality is not supported out of the box. Still, it could be implemented with custom code. RadChart renders an image in the page. You can find the coordinates of the clicked point in the image and knowing the PlotArea Margins
you can calculate the position within the PlotArea. Then you will be able to calculate the logical units which correspond to that position. You can check this article
for more details on obtaining the coordinates of the mouse in an image.
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