I’ve never used it directly, and mostly relied on more practical projects with standard visualisations based on D3.js… the visuals look good though (but requiring an awful lot of JS), still worth investigating some day. This is what the site says:
D3.js is a JavaScript library for manipulating documents based on data. D3 helps you bring data to life using HTML, SVG and CSS. D3’s emphasis on web standards gives you the full capabilities of modern browsers without tying yourself to a proprietary framework, combining powerful visualization components and a data-driven approach to DOM manipulation.