OAuth Gitgraph permit to connect you with OAuth to the Github API.
Your github profile > Settings > Developer settings > OAuth Apps > New OAuth App
This application read 3 SERVER variables :
$_SERVER['CLIENT_ID'];
$_SERVER['CLIENT_SECRET'];
$_SERVER['CALLBACK_URL'];
This application set two cookies name
and token
which authenticate you with the Github Api. The domain chosen for the cookies is the one of $_SERVER['CALLBACK_URL']