How do I improfile a .fig matlab figure using improfile(I,xi,yi) ?

It displays a null graph when I do this. If I save the figure as a .tiff file or any other image format and then improfile, it does give me an RGB separated value graph which is of no use to me. However if I just improfile freehand on the .fig file, it does give a good graph with a single pixel value line and no RGB separated value. Thanks