John Resig (one of the creators of JQuery) has ported the Processing visualization language to JavaScript using the Canvas element

http://ejohn.org/blog/overview-of-processing/
http://developer.mozilla.org/en/docs/HTML:Canvas

includes pie chart

<canvas> is a new HTML element which can be used to draw graphics via scripting (usually Javascript). For example, it can be used to draw graphs, make photo compositions or even perform animations.

Mozilla applications gained support for <canvas> starting with Gecko 1.8 (i.e. Ff1.5). The element was originally introduced by Apple for the OS X Dashboard and Safari. Internet Explorer does not natively support <canvas>, but a page can effectively add support for it by including a script from Google's Explorer Canvas project. Opera 9 also supports <canvas>.

overview by John Resig