All 3 Uses

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

Here is a famous story called The Monty Hall Problem which I have included in this book because it illustrates what I mean.
Chpt 101 *

illustrates = helps make clear
The deerstalker hat was invented by a man called Sidney Paget, who did the illustrations for the original books.
Chpt 107 *

illustrations = drawings
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

illustration = something that helps clarify or demonstrate

Definitions:

(1) (illustrate as in: as illustrated by this example)
to help make clear -- typically by example
(2) (illustrate as in: an illustrated children's book)
to draw pictures (or provide photographs) to accompany a book or other writing
(3) (meaning too rare to warrant focus)