You’ve seen the phrase “Gnuplot is a freely available plotting utility” several times if you didn’t just skip directly to this section. While Gnuplot is indeed a freely available plotting utility, it is also a command-line driven tool. It can be used to generate publishable graphs and charts with minimal effort. Sure, it takes some time to figure out the syntax, and yes you’ll probably make the same graph 20 time to make it “just so,” but would you rather take that information from one project and edit a script for the next, or reinvent the wheel each time using Excel? You’ll take the free command-line tool? Thought so.
Gnuplot’s documentation is very thorough. Information for v5 can be found here.