Your possibilities aren't mutually exclusive: There are many unminified javascript files floating around that are not OSS.

> Is this OSS

Unless the author releases it under an open source license, the answer is no

> am I going to have to dig around minified JS to mess around with it?

The unminified source is available at http://learnforeverlearn.com/usbudget/dist/usbudget.js

The source data is available at http://www.whitehouse.gov/omb/budget/supplemental (note that XLS files are the normative versions)