Ahhh, I see 
The display loop isn’t blocking by default, since most people run code in the repl or vscode, to work on things interactively.
So if you run julia schelling.jl, it will just quit as soon as the script run through.
I recommend working with the REPL / VSCode (simplest form would be to start the julia repl and run include("schelling.jl")).
If you really must run this as a script, you can do wait(display(figure)).
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