All 3 Uses

hypothesis
in
The Curious Incident of the Dog in the Night-time
(Edited)

And if you drew a graph of how many frogs there were in the pond, it would look like this (but this graph is what's called hypothetical, which means that the numbers aren't the real numbers, it is just an illustration).
Chpt 151

hypothetical = based on something that is seemingly reasonable, but unproven
And when X is between 1 and 3, the population gets bigger and then it stays stable like this (and these graphs are hypothetical, too).
Chpt 151 *
And if something is nearby you can find it by moving in a spiral, walking clockwise and taking every right turn until you come back to a road you've already walked on, then taking the next left, then taking every right turn and so on, like this (but this is a hypothetical diagram, too, and not a map of Swindon).
Chpt 179 *

hypothetical = something temporarily treated as true to advance a discussion or to further investigation

Definitions:

(1) (hypothesis as in: a study to test her hypothesis)
a seemingly reasonable, but unproven idea or explanation based upon known facts

In casual conversation, theory is a synonym for hypothesis, but a scientist would say that a hypothesis needs to pass rigorous tests before it could be accepted as a theory.
(2) (hypothesis as in: assume as a working hypothesis)
something that may or may not be true, but is temporarily treated as true to advance a discussion or to further investigation

This sense of hypothesis is typically seen in the form, hypothetical, or hypothetically.
(3) (meaning too rare to warrant focus)