I just checked this thread as well as the other one you mentioned. I believe my colleague Vladimir has provided an example which matches your description. Namely, it shows how to trigger an action (updating a label) upon mouse over a point chart item. I have attached an updated VB.NET version of his page to that thread, so you can check it.
In this thread, you refer to a line chart with 3000 items, where you need to show the exact value for a given point of the line. This is the functionality, which is not available in RadChart. Still, RadChart can show tooltips for line pointmarks and the mechanism is exactly the same as shown in the example I have attached to the other forum thread
you have started. This is also shown in this online example
. You can freely remove the RadTooltipManager if you do not need it.
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