You can use this for any graphs, diagrams, pictures, tables, illustrations, non-artistic photographs, which you have referred to, but not reproduced in your assignment.

You may refer in your text to a graph, table, map, photograph etc which you have seen in a book, journal article etc.

In this case the sentence in your text should make clear the type of illustration you are referring to.

For example:

... Donnan's graph(1) shows that the majority of patients ...

A page reference and figure number, table number etc (if there is one) will be included with the reference in your reference list at the end of your work.

The reference will be the appropriate one for a book, journal article, web page etc, depending on where you have seen the illustration.