Homebrew formula not maintained by anyone (Installation Instructions)
https://git.skewed.de/count0/graph-tool/wikis/installation-instructions
instructions for homebrew install currently are:
brew tap homebrew/science
brew install graph-tool
But the homebrew/science list was deprecated. It appears the graph-tool formula was migrated to brewsci/science. But submitting an issue to them asking them to fix the tap results in this message:
This tap is not maintained. Please consider opening a pull request to migrate this formula to Homebrew/core or a different tap within the Brewsci organization. Please open an issue if you are interested in creating and maintaining a new tap within the Brewsci organization.
Hosting Your Own Tap
Anyone can host their own tap. See Interesting Taps & Forks and How to Create and Maintain a Tap