My earlier answer touched on sources for making bubble charts, but I need to add more on how to get the data for those charts. A possible start might be the Census Bureau, which gets 4 hits for: inequality debt "time series" (use the tiny search box at upper right); and 15 hits for: inequality 1970s. You might do well to skim quickly through the list of about 150 for: inequality "time series", in case they might have many different words implying inequality.

You can also try search.usa.gov, which gets "about 322" hits for: Inequality debt "time series" 1970 2012. But note that this Bing search engine does a smart extract of what it finds most relevant - if you wait a minute or 2, that list drops to 35 or 36 hits. Compare a Google search for: Inequality debt "time series" 1970 2012 site:gov , which gets 168 hits. Note that Google's site:gov limiter will get different results than search.usa.gov - partly because that site includes STATE as well as federal sites and documents, including ones with the old URLs like ny.us, fl.us, ca.us, etc. In Google you can also limit to site:edu , which will get some free research reports, preprints, etc. - very different from the results in scholar.google.com, which is mostly commercial subscription journal articles, that .require you to be on campus at a subscribing university, or at least logging in via a student/staff password to their proxy server.