If this setupHook is necessary for tests, then users won't be able to use tikz outside nix-shell, right? To fix this, replace the hook with makeWrapper.

The problem is that gnuplot doesn't need TEXINPUTS set, but instead latex will need to know how to find gnuplot's tex files when building the output created using the tikz terminal.

That doesn't mean there isn't a cleaner way, which would be welcome! :)

I have not tested this (except that it evaluates), but you can try:

-stdenv.mkDerivation rec {
+lib.fix (self: stdenv.mkDerivation rec {

-    ++ lib.optional withTeXLive (texlive.combine { inherit (texlive) scheme-small; })
+    ++ lib.optional withTeXLive (texlive.combine { inherit (texlive) scheme-small self; })

-    configureFlagsArray+=(--with-texdir=$out/share/texmf-nix/tex/latex/gnuplot)
+    configureFlagsArray+=(--with-texdir=$out/share/texmf/tex/latex/gnuplot)

-}
+})

EDIT: this is incorrect, it will not evaluate with withTeXLive.

Hmm, if runCommand "tikz-plot" succeeds and it needs TEXINPUTS, then the wrapper should be an adequate replacement for setupHook, and it will also support users outside nix-shell. However, I'd prefer my snippet if it works.

gnuplot does not invoke tex-- and so setting TEXINPUTS in the tex we pass it doesn't help. Honestly I'm not entirely sure what purpose passing tex as a buildInput does, I'm not sure it's used at runtime.

The workflow is instead that gnuplot, when using the tikz terminal, generates output that is "incomplete" and needs gnuplot's generated files to be properly build using texlive.
So when the user includes this output in a document (tikz-pdf generates a basic example of this) the tex used needs to know how to find these pieces, hence the setup-hook.
If a wrapper were used, it would need to wrap tex or something, I think.

tikz-plot doesn't need any tex bits, or TEXINPUTS to be set, but rather it's that TEXINPUTS is how the tex in tikz-pdf knows to resolve the gnuplot tikz package/extensions.

At first I wanted to do something like:

texlive.combine { inherit (texlive) scheme-small; inherit gnuplot_full; }

For this purpose (the tex in tikz-pdf, not input to gnuplot build) which maybe would remove the need for the setup-hook and should work outside of nix-shell ... if it evaluated O:).

Unfortunately that's not really how texlive.combiine is meant to work, so instead of faking a texlive package for gnuplot or something I went the setup-hook route.

Make sense?