I've explore a bunch of JS libraries in the past - here are some thoughts:
Flot and Flotr are definitely past favorites, and its usage has felt the most natural
PlotKit (http://www.liquidx.net/plotkit/) - MochiKit-based, visually nice but not as well documented or, apparently, nearly as extensive as some of the other options
RGraph - an HTML5 canvas library, but pretty basic and definitely easy the least visually appealing
Highcharts definitely looks like it has the nicest animations of any of the previous ones, and also definitely has the best documentation / demo gallery. Thumbs up - definitely coming this way the next time I need a nice graphing library!