This is awesome. I hadn't heard of D2, but I love the idea that I can create my charts directly in Neovim in the terminal to get a rough draft, and do a final render with a pretty picture. I will be playing with this shortly.
D2 already supports a live-updating HTTP server serving a live-updating .svg, so if you tile a browser with vim, you can already get live-updates. I discovered D2 about a year ago, and I use it for all of my diagrams now.
There's a sort of elegance to ASCII rendering.