Charts.module
A drupal module with API that supports High Charts and Google Charts libraries. Views integration
project/
code: https://www.drupal.org/project/charts
docs: https://www.drupal.org/node/2423033 + https://www.drupal.org/node/233753
examples:
Google Charts
docs: https://developers.google.com/chart/interactive/docs/index
examples: https://developers.google.com/chart/interactive/docs/gallery
Charts.js
6 chart types
nice animations; very tin js, leverages HTML5 canvas + cross platform fallback
project: http://www.chartjs.org
code: https://github.com/nnnick/Chart.js
docs: http://www.chartjs.org/docs/
examples:
D3
good for inventing things
project: http://d3js.org
code: https://github.com/mbostock/d3
API: https://github.com/mbostock/d3/wiki/API-Reference
examples: https://github.com/mbostock/d3/wiki/Gallery
High Charts
free for non-profits
supports line, spline, area, areaspline, column, bar, pie, scatter, angular gauges, arearange, areasplinerange, columnrange and polar chart
also maps with touch support
project: http://www.highcharts.com
code: https://github.com/highslide-software/highcharts.com
docs: http://www.highcharts.com/docs
examples: http://www.vitalsigns.mtc.ca.gov/transit-ridership#chart-2