Diagram, but it could be part of a diaphragm (5)

I believe the answer is:

graph

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'diagram' is the definition.
(I know that graph is a kind of diagram)

I don't understand the rest of the clue.

(Other definitions for graph that I've seen before include "Drawing showing quantities plotted on axes" , "Diagram showing the relation between variables" , "Visual representation" , "Plotted diagram" , "Mathematical diagram" .)

I've seen this clue in the Evening Standard.

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