PlotKit - Javascript Chart Plotting. 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. PlotKit is fully documented and there is a quick tutorial to get you started. Top 10 custom JavaScript functions of all time. Wednesday Nov 30 2005 UPDATE: For anyone who lands on this article months after the fact, there is now a podcast entry about this article reviewing each and every function.
If there was ever a universal common.js shared among the entire develosphere, you'd fine these ten (plus one bonus) functions. It would be the swiss army knife no developer would go into production without.