I just realized that "Volume of open changesets" at http://korma.wmflabs.org/browser/gerrit_review_queue.html is not showing "How long is the Gerrit review queue over time?" -- that is, the amount of "Pending" and "Waiting for review" changesets every month.
What I deduce it is being shown is some kind of accumulation over time of the changesets *currently open*.
We should be able to see whether the queue is increasing or decreasing over time. Very much like in http://korma.wmflabs.org/browser/bugzilla_response_time.html we can see whether our response time decreases or not every month.