Best open-source visualization frameworks?

I'm building a Rails app and would like to visualize the size of a users folders based on how many files are inside of them. I'm considering giving D3.js a try, but am not sure if I can have it dynamically update as files are added to each folder. I'd like to be able to input my JSON data (which is something D3 does do).

Unless D3.js is the best choice, any other good alternatives?