制作使用JSON作为标记的流程图库的建议

So recently at work I have been tasked with creating a Flowchart library. I would just like to get some general suggestions on how I should approach this. We use mongodb for persistence, and Tornado Web Server as a backend. Basically what I need is to create a library in javascript that has a means of importing JSON in order to display a flow chart. This flowchart will definitely not be trivial, and I will require the ability to model loops, conditions, etc. Basically everything that a standard UML activity diagram would be required to model. I plan on using Raphael to draw the vectors connecting each node on screen. I guess my biggest question is, how would I model loops and conditions in JSON? Are there pre-existing open-source libraries that do what I am suggesting to use as reference?So recently at work I have been tasked with cre