The blog mentions a Graphing Calculator. Not sure if it shares code, but macOS still ships with an app to draw graphs, Grapher.app
Grapher.app is different from Graphing Calculator. It came via an acquisition. All the details are here if you want to read the backstory (assuming the info is correct: https://en.wikipedia.org/wiki/Grapher)
The app on your Mac today isn't a rewrite of the legendary Mac OS 7.2.1 Graphing Calculator, but an acquired app based on Curvus Pro introduced in OSX Tiger.
The first one has a legendary backstory. 2 devs snuck into Apple after their project was canceled: https://www.pacifict.com/Story/
Curvus Pro: https://macintoshgarden.org/apps/curvus-pro