Please note that the RadChart is our old charting component which has many know limitations. This is why decided to create new charting components with better implementation and performance. Also the new RadChartView is very flexible and easy to set up. I suggest you to use the RadChartView instead of the old RadChart. In addition you can take a look at the RadChart vs RadChartView help article.
As you can read in the above mentioned help article the new chartview doesn't support bubble series out of the box. However, you can use ScatterPointSeries with a custom PointTemplate this way you can mimic the look and feel of a bubble chart.
Also the new charting has a ToolTipBehavior and ToolTipTemplate which you can use to control how the tooltips in your series should look.
If I understand your requirement correctly you want to attach a handler on the tooltip opening. If so you can attach to the DataContextChanged event of the parent element in your ToolTipTemplate. Please let me know if I am missing something. If this doesn't help, can you give me more information about your scenario and what exactly you want to achieve. Thank you in advance for the cooperation.
In addition I attached a sample project with the described approach. Please give it a try and let me know if it helps.
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