The chart displays total Twitter followers of Democratic candidates who have more than 500K followers (this is the bubble size,) and how little or how much the candidates' followings overlap . You can find it in a story by Gus Wezerek and Oliver Roeder. Here's what they did:
The people following candidates on Twitter are those who want to receive a steady stream of information about at least part of the 2020 campaign. Understanding how that tribe operates can tell us something about an influential slice of the electorate. So off our web-scraper went, dredging up every follower of the 20 Democratic presidential candidates who FiveThirtyEight considered “major” in early May, when we ran our script.1 The result was a data set with almost 20 million entries, which you can download on GitHub.That's right; the data is available on GitHub, where the authors authors also wrote: “If you use this data and find anything interesting, please let us know. Send your projects to @guswez or @ollie.”