$ brew cask install xquartz
$ brew install gnuplot --with-x11
XQuartz.appを起動しておいて、
$ DISPLAY=:0.0 gnuplot
G N U P L O T
Version 5.2 patchlevel 5 last modified 2018-10-06
Copyright (C) 1986-1993, 1998, 2004, 2007-2018
Thomas Williams, Colin Kelley and many others
gnuplot home: http://www.gnuplot.info
faq, bugs, etc: type "help FAQ"
immediate help: type "help" (plot window: hit 'h')
Terminal type is now 'x11'
おっけー
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