[kian]

I think, in this case, we are seeing semantic a relation between spectra of light (in terms of overlapping sets of frequencies) and producing a 'graph' of the spectrum of rotational frequencies in ink. Anyone know with any more certainty?

[kd5bjo]

Traditionally, “X-graph” refers to the device which produces an “X-gram,” a physical record of some sort (e.g. telegraph/-gram, electrocardiograph/-gram, etc.)

[kian]

I agree :) It seems like the artist/inventor may have at some point missed that finer linguistic point and attached instead to an alternative meaning of the word 'graph' :)