Install XQuartz for X11 on macOS

Is it possible to install XQuartz on macOS using .circle/config.yml?

I expect so. I’ve not used the Mac build servers at CircleCI, but I assume commands are run in the Bash environment. From there, I expect you have access to MacPorts and Home Brew. Can the piece of software you mention be found in either of those? If not, how does it get installed on a local machine?