D3.js is a JavaScript library for manipulating documents based on data. D3 helps you bring data to life using HTML, SVG and CSS.
d3-rails provides D3 for Rails 3.1.
d3-rails comes with version 3.1.6 of D3.js.
Add this line to your Gemfile:
gem "d3-rails"
Please note that D3 is provided via the asset pipeline and you do not need to copy their files into your application. Rails will get them from d3-rails automatically.
Then add it to your manifest file, most probably at app/assets/javascripts/application.js:
//= require d3