It's a beeswarm plot, linked in the footnotes: https://bl.ocks.org/mbostock/6526445e2b44303eebf21da3b6627320 https://bl.ocks.org/mbostock/6526445e2b44303eebf21d

It's a beeswarm plot, linked in the footnotes: https://bl.ocks.org/mbostock/6526445e2b44303eebf21da3b6627320 https://bl.ocks.org/mbostock/6526445e2b44303eebf21da3b662732... As such, the x-axis is a metric about the population, not the individual. I happen to use them a fair bit as I find them easier to intuit density from than the alternative, which is to plot on a single line with transparency.