What would be interesting is to compare information found in the first graph, e.g.
* size of the 'stair-step'
* comparison of the stair size with the release period length
* stabilization (e.g. inclination angle of the slope after the stair)
* average amount of reviews per change
with external information (perceived or computed stability, number of security issues
introduced in a release, holidays)
From my readings of the graph
the last 2 kernels have very steep merge. 2.6.19 started merging late
2.6.22 has had more late merges than its successors
2.6.20 seems to have stabilized early
2.6.16 seems to have had a bad stabilization