Then how do you deal with non-SI units, like money?
For currency conversion, it's a lot more tricky because there's many ways to convert. Eg; if I want to go from CHF to USD there's a lot of CHF->USD volume and there are market rates for that exact conversion. But between eg; SGD (singapore dollar) and Peruvian Soles (PEN) the market is a lot smaller so it may be actually beneficial to do SGD -> USD -> PEN.
Typically this means ingesting a feed of real time prices + any fees and then doing a limited-hop graph traversal.
the base unit is the us dollar
this is arguably incorrect but inarguably useful
from the man page:
> The units program database includes currency exchange rates and prices for some precious metals. Of course, these values change over time, sometimes very rapidly, and ‘units’ cannot provide real-time values. To update the exchange rates, run ‘units_cur’, which rewrites the file containing the currency rates, typically ‘/var/lib/units/currency.units’ or ‘/usr/local/com/units/currency.units’ on a Unix-like system or ‘C:\Program Files (x86)\GNU\units\definitions.units’ on a Windows system.
US Dollars would be connected to US Cents in the graph, and Pounds Sterling and New Pence would be connected to each other too, but those two subgraphs would be entirely disconnected from each other and from any SI unit like distance or mass.