Protovis - Visualization for JavaScript via Canvas

Protovis is a JavaScript library for visualizing data using the canvas element. Custom views of data can be composed with simple graphical primitives like bars and dots. These primitives are called marks, and each mark encodes data visually through dynamic properties such as color and position.

Although Protovis looks complicated, it is very easy to understand (specially for JavaScript users) & has a straight forward logic which offers a flexible way to create any type of visualizations. To see more, you can check the API documentation (very well-documented) & examples built with it.