What is your preferred way to produce charts in a Ruby on Rails web application?

Google Charts is an excellent choice if you don’t want to use Flash. It’s pretty easy to use on its own, but for Rails, it’s even easier with the gchartrb gem. An example:

GoogleChart::PieChart.new('320x200', "Things I Like To Eat", false) do |pc| 
  pc.data "Broccoli", 30
  pc.data "Pizza", 20
  pc.data "PB&J", 40 
  pc.data "Turnips", 10 
  puts pc.to_url 
end