An even earlier tool: Pic, which had its paper released in 1981. It belongs to the troff ecosystem of tools. I'd be interested to know about other tools from this era!

Paper: https://dl.acm.org/doi/10.1145/872730.806459

Mention in troff doc: https://www.troff.org/prog.html#pic

Wikipedia page: https://en.wikipedia.org/wiki/PIC_(markup_language)