There's graph-easy, which generates ascii by default but can also do box chars, and even SVG and png, as well as generating graphviz and other output. It is not WYSIWIG---you feed it a description of a diagram:

   echo "[ Berlin ] -- train --> [ Bonn ] [ Bonn ] --> [ Berlin ]" | graph-easy -as boxart

resulting in

    ┌───────────────────┐
    ∨                   │
  ┌────────┐  train   ┌──────┐
  │ Berlin │ ───────> │ Bonn │
  └────────┘          └──────┘

https://github.com/ironcamel/Graph-Easy