Have you got the various loss terms in analytical (or
vector) form with appropriate indices?
Efficiency is 1 minus the sum of all ineffiencies (loss
terms). Conduction loss in each switch for its duty
factor, which in turn follows step-down ratio; switching
losses which depend on frequency, VIN and VOUT;
internal steady state losses; input and output filter
losses - and so on.
When you have the formula or array-map then the
process of plotting is like any other, I'd suppose.
Hopefully that is not the question, how to press
the right button.