Send me a PM with your Piwik URL/login/password, and phpinfo. I'll take a look, and advise what I find and/or if I require more information/access.

OK, will do. I just found out that it could be a general problem in Piwik since the clickheat plugin has the exact same problem. I've just checked the generated png.

Something else that might be related. I've switched to French and the Flash country map doesn't load anymore: "error while loading data (Markup in doucment"

Ho do you PM on here? I've tried the forum as well, but couldn't find a way either...

More broken things when using French. I tried to create a message and got an endless spinning wheel instead of the error message (my password was too long).
There must be some strange encoding issue lurking somewhere...

*create a user

This is sounding specific to your environment (since you're the only one experiencing a problem).

In the forum, there's a "Private Messages" link at the top of the screen, and a "[PM]" link next to the poster's name.

Thx. I wasn't logged in...
Details sent.

In making your changes, your text editor saved the file with a Byte Order Mark (EF BB BF) at the start of the file. Since PHP doesn't ignore the BOM, it passes these characters through in the output causing the broken sparklines. Using a different editor or stripping out the BOM manually should resolve the issue.

You are correct. I edited the GeoIP plugin and it was breaking all the generated PNGs of the site.
Thank you very much for finding the solution.