What it shows is the number of active developers per month for the last three years. Now, these are not peripheral transients like myself, but core members, with commit rights and everything. As you can see, the project had started to slump down a bit, but for the last year or so is catching speed. And they just launched a new release, which is good.
The original post can be found here: https://anterence.blogspot.com/2010/05/ant-activity.html