I am in school at the moment and do not really have time to read through this discussion, but "wherefore" is an archaic way to say "why" and is an adverb -- an interrogative adverb -- and should be diagrammed as I think Konderosi said.
His images are blocked for me here in school.
Thanks, Kondorosi, for restoring my self-confidence. "Wherefore" (an adverb) does, indeed, "modify" the full verb "are" ( = exist). In other words: Romeo, what is the reason for your being here? Am I correct? (Please say that I am!)
The sense of Juliet's question is "Why do you have to be Romeo (and a Montague). Why couldn't you have been someone else?"
It has nothing to do with "where are you, Romeo?"
In the Romance languages this would be shown by the difference between the verbs (e.g.Spanish) "estar" --"to be" in a changeable way and "ser" -- "to be" in a permanent way.
"O", "Romeo" and "Romeo" are an interjection and nouns of direct address.
The third "Romeo" is the predicate nominative. "Art thou" is archaic for "are you". "Wherefore" is an interrogative adverb.
The scene goes on with the couple trying to believe that his name (and his subsequent part of the enemy family) does not matter.
"A rose, by any other name, would smell as sweet" says Romeo. The "as" in that sentence demands quite a few understood words to make it diagrammable --"as sweet" (as it would be if it had some other name) -- sadly not applicable to their family names.
Amazingly the human mind can deal with that much ellipsis -- and Shakespeare is full of it (or empty, in a sense).
"Wherefore art thou Romeo?" is the same structure as "Why are you a farmer?" (I am only allowed to marry a teacher).
Earlier when Juliet finds out that Romeo (with whom she's already fallen in love) is a Montague she says this sweet speech:
"My only love sprung from my only hate. Too early seen unknown and known too late."
Those two sentences can be diagrammed by supplying "is" in the first one and "He was" in the second.
PS I LOVE discussing such wonderful words!
Juliet is not asking why Romeo has to exist, but why he has to be of that family. Why couldn't the same guy have had some other name? e.g.
"O, Romeo, Romeo, why are you not Joe?"
I am pretty sure that I never said that "wherefore" is a subjective complement. I can't imagine why I would have said that. It is an interrogative adverb and modifies the verb "art". The subjective complement, which in this case I call a predicate nominative is "Romeo".
Maybe I didn't look back through the comments closely enough.