This sounds like a neat idea, one I would like to explore as well. I might be over-simplifying, but I think what you want is to create a synthetic currency based on the movement of 1 active currency as the core (say EURUSD). All synthetic currencies would start from the same value as your core so that when plotted, they would all start from the same point and then diverge accordingly.

In that way, each synthetic currency could be based on the percentage moves of its actual currency, but applied to the synthetic currency, which is based on your core.

Like this:
EURUSD 1.3749 --> 1% move up = 1.3886 --> 2% move down = 1.3608
sGBPUSD 1.3749 --> 2% move up = 1.4024 --> 1% move up = 1.4164

That would allow you to to see divergences, and know to what degree. You might need some sort of special handling in place for JPY currencies I think.

There are some examples of building synthetic currencies in the manual (search synthetic)