For Wikipedia the number of editors is considered to be a very relevant statistic. We do not have those numbers at translatewiki.net. What we do have is the number of editors on a day.
The graph shows that among other things that our summer rally did not only get us more localisations, more people were also the result. At this moment there is an influx of new people interested in localising OpenStreetMap. The result is that some of the languages are now completely localised while others have started.
At the moment Brion is implementing LocalisationUpdate.. I wonder how this will affect the statistics in the days to come..