ChartsGmane gathers lots of statistics that may or may not be interesting to you. Perhaps the most useful bit is a chart that shows the activity per mailing list. It might give an impression of how the interest in the subject covered has changed over the years.
You may wish to carry these charts on the project web pages, so I've made it possible to customize these charts to some degree. Here's the standard look:
The parameters that can be changed are:
Values avaliable for smooth are exp, hann, none, outliers, average.
The data is strongly cyclical (on a weekly basis -- people post less on weekends), so just averaging with outlier removal looks best, I think.
But just use the version you like best.
If you need the raw data, the group information page has that.