Sequence Diagram generator for Ruby or Ruby on Rails

I know about gems rails-erd or railroady. which can generate UML diagrams for a rails application. Add any one to your Gemfile and generate a uml diagram

gem "rails-erd"

In terminal run

ubuntu:~/rails-app$ bundle install
ubuntu:~/rails-app$ bundle exec erd

or

gem 'railroady'

In terminal run

ubuntu:~/rails-app$ bundle install
ubuntu:~/rails-app$ railroady -o models.dot -M 
ubuntu:~/rails-app$ dot -Tpng models.dot > models.png