In LV 5.1, your best shot is to get a full panel image, crop it (array subsets) and export the portion you want. You would have to calculate the panel position of the graph, but that is possible with the "Application Control" VIs. The best choice is to use LabVIEW 6.0. In 6, you can create a reference to the control (the graph), wire it to an invoke node with the method "Get Image". This returns an image of just the graph (or any control, for that matter).