I’ve tried both ways (deleting tmp and after this installing rc4), but none of them are solving the problem. The chart of ecommerce graph has still a black background and grey lines.
Additionally after upgrading (I think not before) I see this error in the log file
mod_fcgid: stderr: PHP Fatal error: Call to undefined method Piwik\Plugin\Manager::findMultipleComponents() in /var/www/…/core/API/ApiRenderer.php on line 101