Hey man, if you accomplish this, you will be my hero. Thanks for tackling it!

Right now I'm working on refactoring git-graph so the git backend can be swapped. It is based on git2 and gitui is moving away from git2 towards gix.

Fonts can be made configurable to display graphs more neatly.
So it would be great if you could make the letter of the join configurable.

kovidgoyal/kitty#7681
https://github.com/rbong/vim-flog
https://www.reddit.com/r/neovim/comments/1ec118n/git_graph_teaser/

The symbols are already configurable. git-graph (not the vs code plugin "git graph") has a configurable option called "style" to change the symbols used.

Regarding the specific symbols you link to, git-graph issue 90 requests the enhancement you propose. Note however, that the proposed symbol-set is larger than the current symbol-set and thus require some changes at the algorithm level. It is a bit more work than simply changing the symbols.

Sorry if I'm wrong.

-s, --style <style> Output style. One of [normal/thin|round|bold|double|ascii].

I thought style could only be set up from these five, since it was written above.

You are right. As a user you can only change symbols by picking one of the predefined styles. There is no support for changing a single symbol (eg. "join") in a style.