Web is amazing, I was trying to find some good tool to create sequence diagrams on ubuntu as my writing is awful and fortunately landed onto http://www.websequencediagrams.com/
All you need to do is write some text in the left pane like
UI->Jersey:Invoke Rest api
Jersey->Spring:Spawn a transaction
and it would generate an image for you in right pane.
Kudos to the guy who created it.