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.