Download and install Gnuplot from http://www.gnuplot.info/download.html.
Ensure that the Gnuplot binary folder is registered in the PATH environment variable so that you can call the Gnuplot executable from any folder.
You may find that Gnuplot reports that the wxt is an unknown terminal type; in which case, use the following command :
Uninstall your current version of Gnuplot and then install the +wxt variant.
port variants gnuplot to list available variants.
sudo port install gnuplot +wxwidgets
Note, after this I had to reboot MacOS to enable Gnuplot to recognize the new terminal types.
Under Linux you need to install both Gnuplot and Gnuplot-X11 :
sudo apt-get install gnuplot
sudo apt-get install gnuplot-x11