It seems similar to http://www.websequencediagrams.com, which plantuml was modelled after.

And that has a million integrations available, including a pretty good emacs-module[1] ;)

[1] https://github.com/josteink/wsd-mode

+1 for plant uml, also for that rose color scheme (for those who rememeber Rational Rose)

That looks much more complete. The syntax is given on the GitHub page but really only covers a very very small subset of UML sequence diagrams.