How do you get the graph plugin for Squirrel SQL?
As per Installation documentation the correct version of each available plugin is always provided in the SQuirreL SQL Client install jar.

Graph Plugin by Gerd Wagner
Installer Category: Standard
This plugin allows you to create and print charts of database tables and their foreign key relations. If you use it together with the SQL Scripts plugin you are able to script tables from within the charts.

You can download it from http://mvnrepository.com/artifact/net.sf.squirrel-sql.plugins/graph/3.2.1