I think the comparison between an IEM graph and a lead sheet is a good one, but I think it's possible to go even deeper into that analogy than you have here. Lead sheets aren't just notes on a page, usually. They'll also denote what instrument is playing (down to the cymbal ornamentation, even, on drum sheets). They'll indicate dynamic cues like accents, crescendos and decrescendos, or temporal ones like ritardandos and rallentados. They'll tell you the chords over which the melody's playing, the tempo at which everything's running, etc. So, you can paint a decent picture if the lead sheet is detailed enough. Similarly, on a graph, you can also glean how forward instruments are based on the prominence of the upper-mids, or how airy and clear it's gonna be based on treble extension, for example. So, I reckon graphs can tell you more than just, say, 10% or 20% of an IEM.
When it comes to the following questions:
I think that's more applicable when you're looking at a specs sheet, rather than an IEM graph. For example, you see that an IEM has a DD, 6x BAs and 4x ESTs. You can then ask the very questions you've posed above. How have their sounds been captured and integrated? What kind of post-production (dampers, acoustic chambers, sound tube shenanigans, etc.) have they undergone? I think that's the more apt analogy. Either way, it's probably one of the stronger illustrations I've seen for an IEM measurement.