Free, open source D3, InfoViz, Processing.js, and Recline.js bring dynamic, interactive -- and jaw-dropping -- data-driven graphics to the Web browser
D3's documentation is voluminous, though it's scattered throughout links on the Web page. The API documentation is excellent, with code sample interwoven among the explanations. While the links in the API docs are to specific functions, everything is organized by activity with each of D3's modules. The Web page also leads to piles of tutorials and presentations on other websites.
You'll have the most fun by exploring the Examples Gallery, which drops you into a treasure trove of sample D3 visualizations. Many of the gallery's examples are accompanied by commentary, as well as annotated source code. You could spend days exploring all the chart types and visualization techniques presented.
A custom, interactive, multidimensional visualization of Titanic survivors created with D3. Clicking on a dimension highlights the "ribbon" for that dimension.