NWR - 2016-05-31

Notes:
1) ABC.TXT (dynamic) is an external data file (dynamic) that is created by an
external program every ? seconds (ie. display dynamic graphs in GNUPLOT).
2) XYZ.??? is a GNUPLOT script file (static) that plots ABC.TXT

Inside a GNUPLOT SCRIPT File (XYZ.???) it seems that GNUPLOT EXIST reacts
like the BATCH DEFINED command

    SET XXX=
    IF DEFINED XXX          ECHO XXX IS DEFINED        (NOT CORRECT)
    IF NOT DEFINED XXX      ECHO XXX IS NOT DEFINED    (CORRECT)

and NOT like the BATCH EXIST command

    IF [NOT] EXIST ABC.TXT     PLOT XYZ.???

YES or NO

If YES, is there a GNUPLOT command to test for an external file in a script?
If NO, what proper syntax is required.

... thanks