PlotKit is a Chart and Graph Plotting Library for Javascript. It has support for HTML Canvas and also SVG via Adobe SVG Viewer and native browser support. It is fully documented and easy to use.

Requirements: HTML Canvas: Safari 2+, Opera 9+, Firefox 1.5+, IE 6
Requirements: SVG: Opera 9+, Firefox 1.5+ (see note), IE6 with Adobe SVG.
Demo: http://media.liquidx.net/js/plotkit-tests/dynamic.html
License: BSD License