I can warmly recommend Mermaid [http://knsv.github.io/mermaid/ http://knsv.github.io/mermaid/]. Extremely straightforward syntax that's readable in plaintext.

I can warmly recommend Mermaid [http://knsv.github.io/mermaid/ http://knsv.github.io/mermaid/]. Extremely straightforward syntax that's readable in plaintext. Output is also super simple with no frills - even though the website doesn't sell that very well.