Markers on graphs

In Google trends there’s this nice feature of showing reported news alongside search trends, for example-

So I was thinking, it would be nice if Piwik had something similar where we could use an external feed to show events through time. A blog could use it to show the dates of blog posts and how this affects visitors, SourceForge sites could use it to show how file releases cause surges in traffic.
I’m not sure if this should be a core feature or an external plugin though. Any ideas?