How to make float window show the(xvalue,yvalue) when my mouse move to the chart?

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Silverlight

Silverlight5

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When my mouse move to the chart area,I can get the real xvalue,yvalue immediately,How can I show then betwenn the mouse(just like a float window)?
I set the tooltip,but when I　drag one point and move,the tooltip can not work!So the other way may be float window,but how the make the float window loacateed arrow the mouse no matter when I drag the point to move or just to move?

The picture I have realise is :https://www.yousendit.com/download/TEhYaUNKMHdveE5FQmRVag[^]


And I want to realise is here: to show real (xvalue,yvalue) around the mouse when the mouse move to the chart no matter I drag the point to move or just to move? Or Do you have a example?Thank you!