I used Sketch (https://www.sketchapp.com/), the graphs of the objects are actually from a built-in template for iOS notifications lol, but it worked well for that purpose.
I bought a book “Teach Yourself C in 21 days” and “OS/2 2.0 PM programming” Because I couldn’t afford anything more, everything in the game I created, Galactic Civilizations, could be found in those 2 books. [...] Let me give you an example – in GalCiv on OS/2, each star ship is actually a full blown window that is of style SS_ICON. So when you move a ship in the game, I’m just using WinSetWindowPos to move the ship X,Y coordinates. There’s no “graphics” in the game per se, just all icons being moved around. That’s because I couldn’t afford any more books than those two and they didn’t cover graphics programming, just icons and window movement.
Source: https://www.stardock.com/stardock/articles/article_sdos2.htm...
By the way, OmniGraffle is also great for drawing graphs on the Mac.