help command displays on-line help. To specify information on a
particular topic use the syntax:
If <topic> is not specified, a short message is printed about
After help for the requested topic is given, a menu of subtopics is given;
help for a subtopic may be requested by typing its name, extending the help
request. After that subtopic has been printed, the request may be extended
again or you may go back one level to the previous topic. Eventually, the
gnuplot command line will return.
If a question mark (?) is given as the topic, the list of topics currently available is printed on the screen.