Hello Olivier,
Look here, I discussed for getting Value with Mouse position :
http://www.syncfusion.com/support/forums/chart-wpf/96123/Update-ToolTip-with-good-valuesI paste here :
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//On serie Mouse Move, I take Mouse Position :
mousePoint = e.MouseEventArgs.GetPosition(ChartArea)
//and then I get the value from Point :
double valx = ChartArea.PointToValue(ChartArea.PrimaryAxis, mousePoint);
double valy = ChartArea.PointToValue(ChartArea.SecondaryAxis, mousePoint);
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It's for cartesian Chart, I don't know if it work for polar, but I hope le logic is the same.
Arnaud.