The 2014 "gnuplot-mode" has the problem of not rendering the greek symbols when asked to by babel, hence, my switch to "gnuplot-mode" 2017.

C-h v gnuplot-program reports

gnuplot-program’s value is "/usr/bin/gnuplot"
  This variable may be risky if used as a file-local variable.
Documentation:
Not documented as a variable.

. . . which is correct, and, yes, as a stand-alone mode it works, i.e., it finds the gnuplot executable and renders the greek letters fine. However, when I attempt it inside a babel gnuplot code block it gives the error of not finding the executable. This is behavior I've seen when babel doesn't see a necessary mode that it requires to work with. This is my supposition/guess. As I recall, when I first tried a babel gnuplot block, it made this same complaint. Then I realized I hadn't installed the gnuplot mode. The problem went away when I installed gnuplot-mode 2014. So again, my educated guess is that babel doesn't see or want to interact with gnuplot-mode 2017, rather, it want to see gnuplot-mode 2014.

I feel like I'm beating this to death. I can simply hand-edit in the diagrams with greek letters done correctly into my org file, i.e., just not do babel gnuplot. Again, gnuplot-mode 2014 in stand-alone will do a "plot file" of the code correctly, but not a "plot buffer" strangely enough. (I'm guessing babel gnuplot wants to do a "plot buffer".) OTOH, this is a bug, i.e., no sane work-around, and we, an advanced species, shouldn't negotiate with or accommodate insects.

So what is the process of keeping babel up-to-date AFA modes interacting with their executables is concerned? Who does this? I can look at the gnuplot-modes and see if I can find anything. But I'm a total noob with big-time Elisp code.

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