I used to use this plugin when I worked with Java EE in Eclipse. It was very handy to write logic diagrams as a part of self documented code.
How can I use it with RubyMine?
1) Install this plugin in RubyMine
2) In the command line install libtool and graphviz
brew install libtool
brew install graphviz
3) open some ruby file and add this code for a test (use block comment to isolate UML diagram from ruby translator =begin =end)
=begin
@startuml
testdot
@enduml
=end
4) Go to menu View -> Tool Windows -> PlantUML
It should display
The environment variable GRAPHVIZ_DOT has not been set
Dot executable is /usr/local/bin/dot
Dot version: ...
Installation seems OK. File generation OK
Another example from official website
=begin
@startuml
:First Actor:
:Another actor: as Men2
actor Men3
actor :Last actor: as Men4
@enduml
=end