graph_libraries_for_rails

jQuery Visualize Plugin: Accessible Charts & Graphs from Table Elements using HTML 5 Canvas
http://www.filamentgroup.com/lab/jquery_visualize_plugin_accessible_charts_graphs_from_tables_html5_canvas/

Bluff is a JavaScript port of the Gruff graphing library  for Ruby.
http://bluff.jcoglan.com/

Open Flash Chart II Plugin for Ruby on Rails - Graphs (we have used this before for interactive charts)
http://pullmonkey.com/projects/open_flash_chart2/

Interactive JS charts for your website (free for personal use):
http://www.highcharts.com/

amCharts: Sexy charts in less than 5 minutes
http://railsontherun.com/2007/10/4/sexy-charts-in-less-than-5-minutes

JS charts (free for personal use)
http://www.jscharts.com/home

Gruff
http://nubyonrails.com/pages/gruff

Google charts
http://code.google.com/p/gchartrb/
http://googlecharts.rubyforge.org/

Flot is a pure Javascript plotting library for jQuery. It produces graphical plots of arbitrary datasets on-the-fly client-side. (sent by @eparreno)
http://code.google.com/p/flot/

A list of 20 JS chart libraries (via @Joel_Hughes on twitter)
http://sixrevisions.com/javascript/20-fresh-javascript-data-visualization-libraries/

http://oesmith.github.com/morris.js/