We use the TimePlot charting library for our systems data analytics graphs. It's mind-bendingly awesome. Really easy to use, as well, though we have to massage the data a little bit before serving it out (because it can't handle quite the density of data our tools gather).
If you're interested in JavaScript data visualization libraries you should check out the JavaScript Infovis Toolkit hosted at http://thejit.org. It was based on five papers regarding different infovis techniques, from Treemaps to Hyperbolic Trees.
these guys are great! a very friendly mailing list too, and neat projects to say the least