Details can be found here on the forum:
Basically once a funnel is created for a given site the graphing of visit fails even though visitor data is collected and can be viewed.
please post in the Funnel plugin, not in a new ticket.
FYI: we've just launched the new Funnels premium plugin! The premium Funnels plugin provides really powerful Conversion Funnels analytics directly in Piwik. Learn more & get the plugin in the Funnels Marketplace page. Check the Funnels User Guide and Funnels FAQ for more information. Looking forward to hearing your feedback!