How to associate a graph object's nodes with image pixel location?

I want to associate the nodes of a graph object with pixel location in an image. For example I might have a tree graph object representing a structure type, say a chair, where the k nodes represent key points of the chair connected by l hierarchal tree edges. I also have the k x 2 pixel locations of those points in an image. How can I associate the two using the new graph object structure, and for example, overlay the graph on the images? say something like:

    imshow(img); hold on; plot(graphObj.XData, graphObj.YData);

In this example, Steve plots a graph structure with XData and YData fields, but when I tried this I get an error message that there are no such public properties associated with the class graph. Any suggestions would be much appreciated.