When an article has edits >= 5/month by logged in editors the article is considered active.
Articles are grouped by the month in which they were created - article cohort.
X-axis (time since the start of the wiki)
Y-axis (article cohorts)
Each row gives the total edit sessions for a given article cohort.
The brush lets you look at the monthly activity of the cohorts. For ex, to see the activity of every cohort in its first month select 1-2 in the filter.