Yes, Ubuntu 19.04. Latest-greatest everything else pertinent as well. Here is are my subscriptions

And so I discover these two when I do list-packages:  

gnuplot            20141231.2137 available  melpa      drive gnuplot from within emacs

and then this:

 gnuplot-mode       20171013.1616 installed             Major mode for editing gnuplot scripts

the former I uninstalled, the latter, as you see, I installed, simply going on the theory that a later something is more up to date. The babel languages page lists this:

Gnuplot gnuplot ob-doc-gnuplot gnuplot, gnuplot-mode

which no doubt means gnuplot the actual program, then gnuplot-mode the Emacs package. So I'm saying the second list-packages offering, gnuplot-mode- 20171013.1616, is working only as a stand-alone mode and not working with org-mode babel (It can't find gnuplot when an org gnuplot block is run.), while the first one, just plain simple gnuplot-20141231.2137 works in org-mode babel, but gives the long-ago solved problem of greek symbol display garbling when run in babel code blocks. (See the links to the github pages.) The confusion is which of the Emacs packages to use -- both having problems. Both modes seem to know where gnuplot is (my $PATH has /usr/bin/ and my gnuplot is /usr/bin/gnuplot), again, they both work fine as stand-alones with gnuplot code files, but, as I'm saying,  gnuplot-mode-20171013.1616 doesn't work with org-mode.

Having glanced at the older gnuplot-20141231.2137 mode's code, it seems to guess version 3.7 if it can't establish which version of gnuplot the user is running. Odd since it does start up an Emacs gnuplot REPL session -- that readily identifies itself as 5.2.6. So I'm guessing this gnuplot-mode-20171013.1616 is not the intended mode after all, rather, gnuplot-20141231.2137. Since there have been more than one stackoverflow efforts on this issue, I thought it worthy of org-mode's attention -- heavy users of greek letters or no.