I use Piwik for analytics, on my sites and my clients, it is a great open source tool, like really great.
Recently I updated a self hosted Piwik by one of my clients, and since the update the “Crunched” data would not appear, empty graphs and empty bars.
The weird thing was that raw data was coming in, and the visitor log was active.
So I looked around, I did not want to do something that might make it worse, and I found this FAQ, and option 4 did the trick 🙂
I guess this is sort of an FYI post, enjoy and spread open-source.