Gnuplot is a plotting program that is still actively used. It was developed by Colin Kelly and Thomas Williams and then released to the world as public domain software. An interesting thing happened because of its wide dispersal--collaborative groups formed to work on new versions of gnuplot.
I became interested in gnuplot in about 1988. At that time Russell Lang of Australia and Dave Kotz at Duke University had each developed variants of gnuplot. Russell contacted me and I put him in touch with Dave. For a few years the three of us were in constant communication as developers.
Over time we were joined with a large number of users and developers. The map here was generated by Gnuplot and shows the communications we had received on our gnuplot mailing list at around October, 1990. I'm the little red spot in Arizona on the map above.
There is no point in tracking correspondences anymore, by the way, since gnuplot is now a Usenet News group: comp.graphics.gnuplot. Hundreds, perhaps thousands, of people now use gnuplot.
My involvement in gnuplot these days is almost non-existent.