The Silverlight framework does not have a built in Clipboard support for images. You can use the System.Windows.Clipboard only to set text. I guess you are using some library for this and perhaps you should ask the library owner for more information.
If I am missing something, please give us more information. It is best if you can create a small project and send it to us so that we can examine what is going on. Note that we do not need your actual application, just the part with the clipboard and the chart.
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