I've looked at the gnuplot mode 2014 github site -- and no issues seem to have been addressed by the owner in recent years. Will try to debug myself.

On Tue, Apr 30, 2019 at 1:09 AM Fraga, Eric <e.fraga@ucl.ac.uk> wrote:
Thanks for the long explanation.  I am using the 2014 version of
gnuplot-mode and gnuplot 5.2. 

gnuplot-mode has a customizable variable, gnuplot-program, which
specifies which command to execute to start gnuplot.  The default value
for this variable, at least in the 2014 version, is simply "gnuplot" so
it will pick up the default gnuplot on Linux (if there is more than one
version installed, I imagine that /etc/alternatives will be used to
identify the default).

If you think the wrong gnuplot is being picked up, maybe customize this
variable?  What do you get if you simply invoke "M-x run-gnuplot"?

--
Eric S Fraga via Emacs 27.0.50, Org release_9.2.3-327-g3375f0