d3.hsl doesnt seem to work with grey scale values

[nonmetalhail]

Seems like whenever I have a color where the saturation is 0% or the lightness is 0% or 100%, I get NaN values in the returned color object.

Easiest to see the various examples here:
https://observablehq.com/d/f7636f7695376a03

But, for example:
d3.hsl('#ffffff') returns {h: NaN, s: NaN, l: 1, opacity: 1}

[mbostock]

This is intended. See:

https://observablehq.com/@d3/achromatic-interpolation

[mootari]

Might be a good idea to add a note to the readme (unless it's already there and I missed it)?