This is really interesting to play around with. There are some heuristics that are being used that seemed odd to me until I remembered that this is supposed to be for diagramming, not necessarily for ASCII art.

For instance, this draws an obtuse angle and a right angle that are not connected to each other:

  _/|__

This simply adds height to the right angle, but does not connect them:

    .   
  _/|__

But this connects both angles together with a sharp point:

    +   
  _/|__

Also, a bit of a nitpick: while I love the amount of examples, I wish they were sectioned off rather than having all of the unformatted text in a single textarea on the left and all rendered images on the right. Trying to scroll the page, I keep scrolling the textarea instead, which breaks the alignment.

Overall, super cool project. I'm looking forward to trying this in real world scenarios soon