There's a big problem with this "20 Years of Linux" graph, and many "XX Years of Linux" graphs as most all show an incomplete history or timeline of events and milestones. The one below for example.
You see, the graph above is missing more than 8 years of milestones.
There is a history before Linus Torvalds started writing the Linux kernel, with many very memorable milestones that are important to the "Linux" operating system.
1983 - Richard Stallman starts the GNU Project. Over the following few years the GNU Project authors GCC, GNU C Library, GDB, BASH, coreutils, binutils, GRUB, etc. Many of the tools that make the Linux kernel and all other software that runs atop it possible, even still to this day.
1984 - MIT starts the "X Window System" now known as the "X.Org Server". X.Org is responsible for graphical point and click interfaces, even still to this day. We wouldn't have had GNOME nor KDE without it. Finally gaining a graphical interface is a very memorable milestone.
1989 - The Free Software Foundation releases the GNU General Public License (GPL) version 1. Without which most of the things in the graph above wouldn't have existed/happened.
And that's to name only a very few. In my opinion, the events above are all important milestones. Far more important than the simple forming of the Linux Foundation. To often do people seem to favor this fake version of history where Linus Torvalds single handedly starts an entire operating system in 1991.