Ilana, It's pretty simple. All you do is a plot/vs For example: yes? plot/vs a,b all the other plot options apply also. Mark -- Mark Verschell NASA/Goddard Space Flight Center verschell@neptune.gsfc.nasa.gov Code 970/Lab. for Hydrospheric Proc. 301-286-2027 Fax: 301-286-1761 Greenbelt, MD 20771