My apologies for inserting this "akshually ☝️", but I'm almost 100% sure that the distro that just didn't make the cut -the one represented in green, right under Manjaro- is openSUSE. It's possible to deduce this from an earlier report of Boiling Steam and its respective graph.

But, perhaps unsurprisingly, I don't ever recall seeing Slackware in any of these.

FWIW, similarly, we can deduce the grey one right below openSUSE to be Gentoo from this report and its respective graph. And, finally, the blue one below Gentoo to be Garuda from the respective graph of this report.

The last distro I was able to identify is Solus. Unfortunately, it's only relevant in the very beginning. It used to be the best performing grey one. See this report and its respective graph.