According to my revision control systems (rcs in those days), I’ve been using gnuplot to make graphs since at least 2002. And I’ve got it set up via a custom gnuplotrc to match the style of the TeX documents I often use the graphs in.

At work we have an Instron 3369 machine for material testing. Recently, I wanted to visualize some tensile test data in ways that I couldn’t get into the test report.