I'd be happy to have your feedback on PR 425; it allows the addition of legends for color scales (and radius, etc) both outside and inside the main figure: https://github.com/observablehq/plot/pull/425 ; see it in action in a notebook at https://observablehq.com/@fil/expose-scales-425 The legend can even be constructed with Plot itself!

We could now link to https://observablehq.com/@observablehq/plot-legends instead?

Excellent solution !!