Draft Markdeep diagrams with a live-updating preview overlaid onto your source code.
Try it out here: https://doersino.github.io/markdeep-diagram-drafting-board/
Your source and settings are continually written to local storage, so – barring browser bugs – they will be restored when you close and reopen the page. Your data isn't sent anywhere else.
With minor changes, you can use this tool to make generative art – see this Gist for the code and some instructions.