This thread looks to be a little on the old side and therefore may no longer be relevant. Please see if there is a newer thread on the subject and ensure you're using the most recent build of any software if your question regards a particular product.
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Please run your app in Visual Studio and at the time that the labels dissapear, do you see the following message in the Visual Studio output console?
A first chance exception of type 'System.ArgumentOutOfRangeException' occurred in PresentationCore.dll
If you can send me an updated version of your repro app that reproduces this issue, that would be a huge help to look into fixing this. (Currently the repro app hangs when that data reaches 1.0 on the X axis)