Nice picture! Do note that d3.interpolateSpectral is not intended to be an accurate depiction of the spectrum, but instead is supposed to be a general-purpose divergent color scale with a sort of poetic name ... so the colors here don't match up at all to wavelengths. Rendering a spectrum into sRGB gamut is tricky. Here was my quick effort a decade ago: https://en.wikipedia.org/wiki/File:Jacobolus_spectrum.svg or there was other discussion at https://en.wikipedia.org/wiki/Talk:Visible_spectrum#Spectrum_images

This is very helpful, thank you! I will look into implementing this.