The official web site for the gnuplot project is www.gnuplot.info. There you’ll find the official documentation, including the extensive and helpful FAQ as well as links (not always current) to other sites and related projects.
Community support is available through the newsgroup comp.graphics.apps.gnuplot and the mailing list firstname.lastname@example.org. There’s also a mailing list for gnuplot developers: email@example.com.
Very helpful, with many examples of more advanced uses, is the “not-so-frequently asked questions” list, maintained by Toshihiko Kawano at the Los Alamos National Lab: http://t16web.lanl.gov/Kawano/gnuplot/index-e.html.
And if you’re curious ...