I haven't had time to dig into it much, but I do know that when you open Data Visualizer diagrams in Visio, they have a special frame around them. These frames have special User-defined cells, and there may even be a few new methods that "do stuff".
John G has a five-part series on Data Visualizer, and I would bet that he digs into some of the structure. It starts here:
Visio Data Visualizer part 1 – the processI don't know how much of a web-api is available for this yet, if any. I think there will ultimately be two levels of online automation:
1. working with static Visio drawings--where users can't edit anything, but Plan 1 users could run some scripting to get additional data or integrate with other web parts.
2. working with the actual online drawing interface.
I don't know if anything exists for #2 yet at all. I just haven't had time to investigate, plus the naming and licensing is so confusing, it has kind of turned me off to the whole thing. But we can't ignore it forever, because "Cloud Visio" is here to stay, I'm sure!