If you’re trying to run rdoc but are getting errors like..
Generating HTML…
Diagrams: ./opt/local/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:27: command not found: dot -Tismap dot/f_0.dot
./opt/local/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:27: command not found: dot -Tismap dot/f_1.dot
..the package you need to install is called graphviz. You can install it using Darwin Ports on OS X like so..
sudo port install graphviz