I worked extensively with Highcharts and Highcharts Stock. Not a lightweight library, but it fit the project requirements and scope.
But whenever possible, I tried to keep it light and hand-code the charts with SVG. Once you get the hang of it, it is pretty fun, and you can move fast.
Charts.css tries to "replace traditional JS charting libraries with a CSS framework".
The documentation is well put together, and the demos are impressive. Nice to see that you can go crazy with the styles. The bad news is that some typical chart types are not yet implemented.
I'm curious if they can come up with a way to use the
table markup to create the pie, donut, polar charts in a sane way. I'm definitely following the development of this.