UPDATE(2019-05-19 Sun): Plan A and B use options no longer available since the recent changes to remove all options from Homebrew/homebrew-core formulae. See Plan C.
If you have the resources, you can try the Qt flavor. You need at least 15GB to download and a long build. Ran out of space on my Macbook Air. Aborted.
brew install gnuplot --with-qt
brew install Caskroom/cask/xquartz
brew install gnuplot --with-x11
Feedgnuplot is handy for plotting data streams realtime.
brew install feedgnuplot
brew install gnuplot
So far so good, but default gnuplot formula uses Qt and the Cocoa plugin could not be loaded:
qt.qpa.plugin: Could not find the Qt platform plugin "cocoa" in ""
QT_DEBUG_PLUGINS
Turns out you can get plugin logs using the QT_DEBUG_PLUGINS environment variable:
export QT_DEBUG_PLUGINS=1
QFactoryLoader::QFactoryLoader() checking directory path "/Users/myuser/homebrew/Cellar/gnuplot/5.2.6_1/libexec/gnuplot/5.2/platforms" …
This led me to find out about the gnuplot/5.2/gnuplot_qt binary.
qt_prfxpath
Getting the Qt prefix can be done by inspecting QtCore's strings:
strings /Users/myuser/homebrew/Cellar/qt/5.12.3/Frameworks/QtCore.framework/QtCore | grep qt_prfxpath
qt_prfxpath=/usr/local/Cellar/qt/5.12.3
Ok so qt_prfxpath is pointing to usr/local/Cellar/qt, while my installation's is at //Users/myuser/homebrew/Cellar/qt. This is problematic and indeed my fault for installing homebrew in Users/myuser/homebrew instead of the recommended //usr/local.
Symlinking did the job:
sudo mkdir -p /usr/local/Cellar
sudo ln -s ~/homebrew/Cellar/qt /usr/local/Cellar/qt
/Users/myuser/homebrew/Cellar/gnuplot/5.2.6_1/libexec/gnuplot/5.2/gnuplot_qt
Success.