plotContrastHistogram

   [cHistN,cHistX] = plotContrastHistogram(data)

 Author: Wandell
 Date:   March 18, 2003
 Purpose:
    Plot or return a contrast histogram of the input data.  These data may
    be in an image matrix format.  Here they are converted to a list of
    numbers and the contrast is computed.  
    If there are no output arguments, a figure is created and the histogram is plotted.  
    If there are two outputs, the histogram values are returned.  These
    can be plotted using

               plot(cHistX,cHistN,'o') 
               bar(cHistX, cHistN)

    or other general routines