Plot.legend with different domains for color and symbol

[Fil]

Trying to generate a legend when the color and symbol axes are both set and have different domains:

Plot.legend({
  color: {
    domain: [1, 2, 3]
  },
  symbol: {
    domain: [4,5,6]
  }
})

This will currently silently output

with no clear indication why neither the colors nor the symbols are visible.

Originally posted by @mwaibel-go in #755