How to draw diagrams for git workflow?

I want to make some workflow graphics. Is there a convenient tool for that?

I found some Git workflow diagrams in Vincent Driessen's Successful Git branching model and Jussi Judin's blog post in response. I'll paste in just a small chunk of Driessen's drawing here to give you an idea what I'm looking for:

I want to create new lovely "cartoonish" drawings of that sort for a project report. I like that they have somewhat "fuzzy" text boxes, pleasant callout shapes with configurable pointers to dots, and colored dots. I wish the editor would make it convenient to re-align pieces. It would be nice if arrows might "move with the dots" to which they point in an "easy" to adjust way.

Can you tell me how it can be done in Ubuntu? I'm aware of dia, which makes less jazzy illustrations.

Please don't say I need a Microsoft Surface to do this :)