The library, in the order it happened
The same papers and the same relationships as the map on the front page, laid out on a time axis. What it shows that the map cannot is the lag: a method appears, and you can see how long it took the next paper to pick it up.
Reading the library…
Papers per year
A thin year here means the library is thin there, not that the field was quiet. The corpus grew outward from a handful of papers along their own citations, so it is densest where those papers were looking.