Cool graphic, but I couldn't quite guess what all the symbolism was supposed to mean... (I'm sure it's easy, once you learn it.)

Does PostgreSQL work like Oracle, storing the table names, column names, data types, constraints (like "primary key", "unique", "not-null", etc), and "foreign key" (join-column) relations in it's own set of distinctly named tables? If so, then doing a suitable set of queries on these tables will give you the data you need to create such a chart, and if you're already using DBI and DBD::Postgres, it should be pretty quick work to cook it up.