In this article, Srividya Sharma provides an introduction to FusionCharts Free, a Flash charting library, and shows how to use it from Ruby.
Next to a complete sample application showing how to implement Flash charts in a Ruby on Rails application, the article also contains a thorough comparison of FusionCharts (Free and Paid versions) with other charting solutions.
Using Flash is just one way of providing charts to the user; InfoQ previously ran an article on using Google Charts with Ruby. While the Google Charts solution offloads the rendering of the charts to Google's servers, the FusionCharts solution allows to render the data on the server and avoid the dependence on a third party.