NVD3

http://nvd3.org/

• Initial evaluation: seems the most mature. Apparently, has bad documentation, and is hard to use without going to its source code.
• • undergoing big rewrite

dimple

http://dimplejs.org/

• Initial evaluation: its main goal is to offer a simple API to create charts

C3.js

http://c3js.org/

• Initial evaluation: highly customizable charting API.
• • tooltip issues in firefox
• • almost no documentation
• • some drawing artefacts

xCharts

http://tenxer.github.io/xcharts/

• Initial evaluation: "designed to be dynamic, fluid, and open to integrations and customization" seems to have a rather complex API though * - last commit 1 year ago
• • no pie chart

Vega

http://trifacta.github.io/vega/

• Initial evaluation: intends to generate data visualizations from a JSON document. Charts can be generated server-side. Has an online editor http://trifacta.github.io/vega/editor/
• • no tooltips

rCharts

http://rcharts.io/

• Initial evaluation: based on other two open-source projects (rVega, and rHighCharts). Creates a layer of abstraction on top of other charting libraries (like NVD3, HighCharts, Rickshaw, Morris, Polycharts, or Leaflet) using the R language.

Rickshaw

http://code.shutterstock.com/rickshaw/

• Initial evaluation: looks similar to NVD3 in its goal of providing an API that allows you to customize the different elements that compose a chart (axis, series, legends, ...)

Dygraphs

http://dygraphs.com/

• • Plot charts only

ChartJS

http://www.chartjs.org/

• Six types of charts
• Canvas based
• Retina-display support
• Supports IE8

MetricsGraphics.js

http://metricsgraphicsjs.org/