I searched in Duckduckgo.com on slackware release history and
found this Wikipedia article
which contained a nice table of data. I used gogeeqie
to graph the data. This made a .svg file which I posted.
The data point for Slackware 15.0 is on the top line (event 35), so it
is hard to see, but if you look closely you can see the vertical line
from the "+" sticking up.