This tap provides formulae with hard dependencies on X11 (which has not been shipped with OS X since 10.7).
To install these you will need to have XQuartz installed with brew install Caskroom/cask/xquartz or from https://xquartz.macosforge.org/.
brew install homebrew/x11/<formula>
Or brew tap homebrew/x11 and then brew install <formula>.
Or install via URL (which will not receive updates):
brew install https://raw.githubusercontent.com/Homebrew/homebrew-x11/master/<formula>.rb
brew help, man brew or check Homebrew's documentation.