Macports installs all ports with
make -w install --prefix=/some/weird/path
In gnuplot that works perfectly for everything but TEXDIR. As a consequence files gnuplot-lua-tikz.sty, gnuplot.cfg, ... end up in the wrong place and macports isn't able to remove them when gnuplot is uninstalled. One can create patches and workarounds, but it would be nice to fix the behaviour in gnuplot's configuration. See also Dan's report at https://trac.macports.org/ticket/32872.
Another related issue is that files should end up under
and not all under tex/latex/gnuplot since they are useless both in plain TeX and ConTeXt if all of them are simply put under latex.