I'm having a problem with the ItemToolTipOpening event. The last value in the line and spline chart (and maybe others) shows the previous item's y value. For an example of the problem, look at the First Look spline chart in the Silverlight demos.
http://demos.telerik.com/silverlight/#Chart/FirstLook
If you mouse over the point mark for 2009, you'll get data from 2008.
http://demos.telerik.com/silverlight/#Chart/FirstLook
If you mouse over the point mark for 2009, you'll get data from 2008.