A possible start might be a Google search using site:edu, site:ac.uk, etc - to find unpublished or recent reports from university departments. For example, Google gets about 20 hits for: "bubble chart" inequality debt site:edu

You can also try site:gov, but first try the search: "bubble chart" inequality debt , in search.usa.gov, to find STATE as well as federal reports, documents, and web sites. I have no luck trying "bubble chart" inequality debt site:edu in Google images, but "bubble chart" inequality site:edu gets about 30. You can certainly try these searches in Google Scholar, without of course the site: edu, since Scholar IS all academic sources (at least we hope so!). But you would need to be onsite at a large university that subscribes to the many journals that Google Scholar indexes - or at least be going through a university's proxy server to get to Google Scholar.

If you are at a large academic library, you can also try commercial databases such as ScienceDirect, which defaults to full text search of all of its journals (all scholarly), and gets 3 hits for: Chart w/50 inequality w/50 debt. But authors often will not SAY if a chart is a bubble chart - you will probably have to look at a lot of charts or figures, to find a bubble chart that is not called a bubble chart. Also, that full text proximity search varies: Ebsco databases such as Business Source or Academic Search would require you to "select a field" TX-All text, and search: Chart n50 inequality n50 debt. JSTOR(default full text search) would say "chart inequality debt"~50. But JSTOR advanced search lets you select search of "caption", and that gets 7 hits for: inequality and debt. The thumbnail images don't appear to be bubble charts, but you may want to try looking in the full text PDFs, in case - or try different words.