If you have questions that are not answered in this guide, there are several places to look for help.
The first place you should look is the gnuplot manual or the online help facility. These contain the same information. The manual has fairly detailed explanations of all the commands and their options. However, the commands are listed in alphabetical order. This is fine if you want to know what a command does or what options go with it, but it's not so useful if you don't know how to do something at all. The online help is menu-driven, but still isn't particularly helpful if you don't know what to look for. (This is one of the motivations for this User's Guide.)
There are many demo gnuplot scripts included in the distribution. They are in the DEMO subdirectory on your PC. There are a great many tips and tricks you can learn from these scripts, so check them out. Also included with the distribution is a file 0FAQ. This file contains frequently asked questions related to gnuplot and their answers. The most updated version of the FAQ is posted to the newsgroup comp.graphics.gnuplot every two weeks.
If you cannot find an answer to your question in the above mentioned documents, you can check out the newsgroup comp.graphics.gnuplot. It may very well be that someone has asked the same question and the answer was posted. If not, you can post your question there.
The official distribution site of gnuplot is ftp.dartmouth.edu. You can find the source of the latest version there via anonymous ftp. There is a contrib section, which contains many extensions/modifications to gnuplot. The Macintosh version of gnuplot is also contained in the contrib section.
The latest version of this document can be obtained by anonymous ftp to picard.tamu.edu in the directory /pub/gnuplot.