> completely dependent on that Software-as-a-Service.

Indeed, that was disappointing for me. People on the look out for a similar tool might want to check graphviz and its emacs mode.

http://users.skynet.be/ppareit/projects/graphviz-dot-mode/gr...

or

http://orgmode.org/worg/org-contrib/babel/languages/ob-doc-d... if you're into org-mode.