The bubbles on the chart represent a specific supplement's claim such as "green tea is good for cholesterol levels." How high the bubble is on the chart is meant to illustrate the strength of the scientific evidence backing those claims.
In explaining how the guide was created, the designers said:
We only considered large, human, randomized placebo-controlled trials in our data scrape-wherever possible. No animal trials. No cell studies. Many of the health claims made by the $23 billion supplements industry are based on non-human trials. We wanted to cut through that...
...We looked at the abstracts of over 1500 studies on PubMed (run by US National Library Of Medicine) and Cochrane.org (which hosts meta-studies of scientific research). It took us several months to seek out the evidence-or lack of.
You can see our key results in this spreadsheet. (It’s the same spreadsheet that generates the interactive image).
Via Boing Boing