Isn't it confusing to call this the "natural argument order", when everything is being converted to the standard financial industry order (if I understand correctly)?  Maybe just call it the "original matplotlib argument order"?  Or is there some larger rationale or citation for calling it "natural"?  (To me, as a non-financial person, it does seem quite natural--but since this is a function for financial people, what seems natural to me is completely irrelevant.)