There is no built in way to do this, but if you put both the xamDataChart and the Legend in a Canvas, you could listen to the mouse down and mouse move events on the legend, and adjust its x-y offset in the canvas as the user dragged the legend around.

I believe some of our samples may give you a good idea of one way you can accomplish this. Some of them feature a floating tool pane that can be dragged around and collapsed, I would recommend checking out how that is accomplished. (e.g. http://samples.infragistics.com/sldv/RunSamples.aspx?cn=data-chart#/data-chart/binding-live-data)

-Graham