I'm running the subversion trunk copy of Piwik  and I've been getting useless graphs for a particular site.
I'm attaching a screen capture.
Get the same bug in 0.1.8. The piechart and the vertical bar graph are displayed as in the screen capture above and the popup just says "undefined".
Do the sparklines show? For example below the graph on Visitors > Overview ?
If they don't show, right click on them and View image, and copy here the error message.
Otherwise send me a link to your buggy piwik with login/pass to access it.
the sparklines work. i sent the link with the login/pass to firstname.lastname@example.org.
Gaxt, I checked your website, it seems to work fine. Closing the ticket, please post more information if you have more.