Surprised by your comment, I took a gander at graphviz; it's about a quarter of a million lines of code if you discount lib/sparse/color_palette.c and lib/edgepaint/lab_gamut.c, which are hundreds of thousands of lines of data values. This is much more than I expected.
If you want something that builds diagrams from textual descriptions, you might want to check out TikZ, which includes a subset of graphviz but also lets you draw anything you want. See https://en.wikipedia.org/wiki/PGF/TikZ. On the other hand, you won't have the rapid visual feedback you get with WYSIWYG drawing editors like FigJam.
If you want something that builds diagrams from textual descriptions, you might want to check out TikZ, which includes a subset of graphviz but also lets you draw anything you want. See https://en.wikipedia.org/wiki/PGF/TikZ. On the other hand, you won't have the rapid visual feedback you get with WYSIWYG drawing editors like FigJam.