...and the following error message appears instead of the graphic :
Fatal error: Cannot use object of type Piwik_DataTable_Row as array in /home/kunden/default/htdocs/piwik/piwik/plugins/Actions/Actions.php on line 480
site has about 2.000 visitors / day.
Another site with much less visitors does not throw this error...
Please send email to matt (at) piwik.org and attach a mysqldump of your database (excluding the archive tables).