If graphael gets into shape it might be decent as well.
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!
I've Googled in the past but haven't found too many great reviews comparing some of these choices side-by-side - most seem to regurgitate the feature sets as opposed to going into the level of detail that would be most helpful for a developer.
I'm working on a graphing app too and trying to decide my pricing.
The best and fully featured graphing capabilities are from Dundas imo (www.dundas.com). They charge about $3000 / developer licence and then $3000 / server that you run it on.
If you priced it somewhere between the two and it had a good feature set I might bite. It would have to support printing though (and generating a PDF would be a big plus)
Email me at firstname.lastname@example.org and I'll put you on our early adopters discount list.
The big problem I see with a project like Highcharts (and other Flot alternatives I've tried) is that features like animation and pretty aesthetics are center stage while performance is typically hugely lacking. We've driven Flot to its limit, and by far I can say that the winner of JS charting APIs is going to be the library that can perform well not just on Chrome but on Internet Explorer too.
We've charted things here that take under 1 second to render in Firefox/Chrome/Safari while taking 10-15 minutes with the CPU pegged (yes, you read that correctly) in IE.
We've gotten around it with a bit of work, but it's still a glaring problem for the APIs, and a glaring problem for projects that use HTML5 canvas, can't use excanvas.js with the Silverlight patch, but do need to serve up JS-powered charts to clients still on IE6/IE7.
As I mentioned, VML is remarkably close to SVG. So there's an obvious solution here.
You can see an example here, which took about an hour to build:
P.S. CustomerFind v2 is no longer focusing on AutoFollow, though the copy on the homepage reflects otherwise [http://customerfind.posterous.com/do-you-sell-a-product-or-s...]
I use a modified version of Flotr, and it works great.
Edit 2: I have a question. Are the charts on the Demo page drawing so slowly because of someone's clever idea of an animation? Or does the library actually draw that slowly?
I would prefer bland but matched colors.
Specifically: I want a JS map library that can read SVG maps and render in a canvas. I think I'm gonna have to build it myself.