DrawIt
http://www.drchip.org/astronaut/vim/index.html#DRAWIT Updated Jan 05, 2018 - Jan 07, 2020 (v15a)
Manual
A plugin for drawing vertical, horizontal, and slanted lines using text. With DrawIt, one may use the number pad to leave a trail of dashes, bars, etc behind. Furthermore, it also supports using visual-block specified regions to draw boxes, ellipses, arrowheads, lines, and to specify floods. New: DrawIt has two new commands, :DIsngl and :DIdbl, that tell DrawIt to draw boxes using utf-8 box characters (single-bar, double-bar). Your current file needs to be using the utf-8 encoding for this to work.
Also available at vim.sf.net where you may rank it. The version here requires vim 7.0!!!