NAME: 
  zplot
 PURPOSE:
  Plot differential refraction as a function of wavelength.
 DESCRIPTION:
 CATEGORY:
  2-D plotting
 CALLING SEQUENCE:
 INPUTS:
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  1991 - Written by Marc W. Buie, Lowell Observatory