NAME:
  rddat
 PURPOSE:
  Display the X and Y value of the cursor in a plot
 CATEGORY:
  2-D plotting
 CALLING SEQUENCE:
  rddat
 INPUTS:
  None.
 OUTPUTS:
  None.
 COMMON BLOCKS:
  None.
 SIDE EFFECTS:
  The X, Y and data values of plot under the cursor are constantly 
  displayed.

  Pressing left or center mouse button, makes a new line of output,
     saving the old.
  Pressing right mouse button, exits the procedure.

 RESTRICTIONS:
  None.
 PROCEDURE:
  Straightforward.
 MODIFICATION HISTORY:
  DMS, Dec, 1987.
  cloned from RDPIX by Marc W. Buie, March 1991.
  98/06/27, MWB, converted from !err to !mouse
  99/05/28, MWB, added kludge fix for operation on a Windows platform.
                   Unless RSI implements  (no line feed) this
                   routine (and rdpix) are not fully useful.