radius legend?
This might be a bit difficult to do (do we want superposed circles? how to chose the reference values? even by hand it's difficult…), and it would in any case need inferRadialRange (#235) to be feasible.
related #23
https://observablehq.com/@observablehq/plot-radiuslegend creates it in a plugin, similar to the colorlegend plugin.

superseded by https://github.com/observablehq/plot/pull/432
+1 I think this would be a good enhancement for plot.
see https://github.com/observablehq/plot/pull/665
Sumanta Pattanaik
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Another tentative here https://observablehq.com/@observablehq/plot-us-bubble-map#radiusLegend
this is Vega-Lite style: