A few months ago, I released a gem called quacky-charts, which could be used to draw pie charts in Rails apps.  Last night, I added a feature which allows you to draw line graphs.  The graphs are still pretty ugly, but the basic features are in place.  Improving their appearance should be pretty easy from here.  More importantly, I made a site for documentation.  Check it out at http://www.quacky-charts.com/.