I've always been really impressed with the look of chart.js. When I was looking for a charting library to use last year chart.js was high up on my list because of how awesome it looked. Ultimately though it just didn't have the option set to allow me to do what needed to get done so I had to go with highcharts. It looks like it's come a long way since then though. I'll definitely keep an eye on it in the future.