The correct default symbol set is only inferred when fill is a channel, not a constant

[mbostock]

For example, this works fine:

Plot.dotX([
  "circle",
  "cross",
  "diamond",
  "square",
  "star",
  "triangle",
  "wye"
], {fill: () => "currentColor", symbol: (d, i) => i}).plot()

But this does not (because it uses the stroked symbols, even though the dots are filled):

Plot.dotX([
  "circle",
  "cross",
  "diamond",
  "square",
  "star",
  "triangle",
  "wye"
], {fill: "currentColor", symbol: (d, i) => i}).plot()

The dots can be made visible by applying a stroke:

Plot.dotX([
  "circle",
  "cross",
  "diamond",
  "square",
  "star",
  "triangle",
  "wye"
], {fill: "currentColor", stroke: "red", symbol: (d, i) => i}).plot()

[Fil]

The bug is only when using currentColor (a constant fill: "black" passes through as a hint).