The graphs are part of a talk, and you miss the spoken explanation here :)
The graphs come in pairs. The second is a zoom-in on the bottom part of the first. You might notice that the lines that are high on the first graph of each set do not appear on the second. The colors might have hinted you to this.
The resource limits esp. in memory. At start, most memory was available. Halfway the long running process, the system also needed (lots of) memory for other processes and started swapping. They tied hashes where about 4 Gb each (4 of them).
Enjoy, Have FUN! H.Merijn