The denominator **is** arbitrary, because they always cancel out.  

It's amusing you're harping on this **necessity** for USD to be in the conversation, when there's provably no logical need for it!  (see the above algebraic proof, and the fucked up state of the global economy, "denominated in dollars") .

Look, you can't just arbitrarily put USD into the chart (in your head) & then complain when folks tell you you're being arbitrarily & stubbornly horny for fiat.