The USD Denominates Everything
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Because apparently it needs to be said,

the only reason you can make the conversion BTC->XMR 
or to whatever

is because you have an underlying UoA denomination to compare them with.

its the US dollar.
it denominates everything.

you are comparing
two USD denominated prices.

the price of one thing.
as measured in USD.
against.
the price of another thing.
as measured in USD.

A comparison of the relative FIAT value of two things,
that is what a price graph is. 
As long as the UoA is the US dollar.