ChartJS provides beautiful flat designs for charts. It uses HTML5 canvas element for rendering. Support for older browsers like IE7/8 is added through polyfill.
ChartJS charts are responsive by default. They work well in mobiles and tablets. With 6 different types of core charts out of the box (core, bar, doughnut, radar, line, and polar area), ChartJS is definitely one of the most impressive open source charting libraries in recent times.
If you are an AngularJS developer, you will definitely find n3-charts extremely useful and interesting. n3-charts is built on top of D3.js and AngularJS. It provides various standard charts in the form of customisable AngularJS directives.
Checkout list of charts built using ns-charts.
Ember Charts is another great open source repository built with D3.js and Ember.js. It provides time series, bar, pie, and scatter charts that are easily customisable. It uses SVG to render charts.
ZingChart offers a flexible, interactive, fast, scalable and modern product for creating charts quickly. Their product is used by companies like Apple, Microsoft, Adobe, Boeing and Walmart, and uses Ajax, JSON, HTML5 to deliver great-looking charts quickly.
Free to try, ZingChart licenses can also be bought at a variety of price-points to suit your business size. Check ZingChart out here.
Highcharts JS is another very popular library for building graphs. It comes loaded with many different types of cool animations that are sufficient to attract many eyeballs to your website. Just like other libraries, Highcharts comes with many pre-built graphs like spline, area, areaspline, column, bar, pie, scatter, etc.
One of the biggest advantages of using Highcharts JS is the compatibility with older browsers like Internet Explorer 6. Standard browsers use SVG for the graphics rendering. In legacy Internet Explorer, graphics are drawn using VML.
While Highchats JS is free for personal use, you need to purchase a license for commercial usage.