Couldn't something like this be done.

Grab the activity of the first 100 users. Starting from highest to lowest. divide the total by 100 and you will have your avage activity. For example: Lets do 10 users.

User1 = 100%
User2 = 98%
User3 = 94%
User4 = 90%
User5 = 85%
User6 = 82%
User7 = 82%
User8 = 76%
User9 = 71%
User10 = 69%

Total = 847 / 10 = 84.7% would be your avarage. Then do something like <background ="act_graph.gif" width="84%">

Cant something like that be done?