This is a plugin using yaml.

see http://yuml.me/diagram/scruffy/class/samples

This is an example use case using yaml:

{{{
#!YumlUseCase
[User]-(Login)
[User]-(Logout) 
(Login)<(Reminder) 
(Login)>(Captcha)
}}}

This is an example class diagram:

{{{
#!YumlClass
[Customer]+1->*[Order]
[Order]++1-items >*[LineItem]
[Order]-0..1>[PaymentMethod]
}}}

This is an example activity diagram:

{{{
#!YumlActivity
(start)-><d1>logged in->(Show Dashboard)->|a|->(end)
<d1>not logged in->(Show Login)->|a|
}}}