The way I use ASCII art diagrams in my source-code is through this simple web service that I hosted:

https://dot-to-ascii.ggerganov.com

I generate the ASCII graph and then paste it in the comments together with a link to the service. This way I can go back later and update it if needed:

/*

Link: https://dot-to-ascii.ggerganov.com/?src_hash=476410d3

                     0.6
                 ┌─────────────────────────┐
                 ▼                         │
  ┌───┐  0.8   ┌───┐  0.1   ┌───┐  0.7   ┌───┐  0.2   ┌───┐
  │ d │ ─────▶ │   │ ◀───── │ e │ ─────▶ │ a │ ─────▶ │ b │
  └───┘        │ c │        └───┘        └───┘        └───┘
               │   │  0.4                               │
               │   │ ◀──────────────────────────────────┘
               └───┘

*/