dimple is a flexible and powerful charting library built on d3.js. Strengths of dimple include multivariate plotting, thorough documentation, multiple chart types, numerous examples, and MIT license.

Here is the description directly from the dimple website.

Simply Powerful

The aim of dimple is to open up the power and flexibility of d3 to analysts. It aims to give a gentle learning curve and minimal code to achieve something productive. It also exposes the d3 objects so you can pick them up and run to create some really cool stuff.