GraphViz is a powerful way of describing diagrams of any kind, using just text. There is no graphical editor, so this may not be the tool of choice for the occasional user. But if you would like to visualise your ideas regularly it is well worth reading more about the language. Have a look at the following resources on the GraphViz website, to learn more than what can be explained on this overview page.
General
Content
Integrations
App links