I've used protovis for an internal graphing framework. It's absolutely, positively, 100% the best visualization tool for the web. The syntax is easy to work wi

I've used protovis for an internal graphing framework. It's absolutely, positively, 100% the best visualization tool for the web. The syntax is easy to work with (once you get over the initial learning curve, which is somewhat steep), and it flies on a modern browser. It's the only framework out there for visualization development that's flexible enough to do interesting things with. I think the only other comparable tool is processing (not processingjs, which is too slow for non-trivial tasks), although processing is focused on pixel manipulation whereas protovis is focused on vector drawing.