Crossword Genius

A number flipped over for part of graph (4)

Ross

I believe the answer is:

axis

'for part of graph' is the definition.
(as in X or Y axis)

'a number flipped' is the wordplay.
'number' becomes 'six' (six is a kind of whole number).
'flipped' says the letters should be written backwards.
'six' in reverse letter order is 'xis'.
'a'+'xis'='AXIS'

'over' is the link.

(Other definitions for axis that I've seen before include "Imaginary line around which a body rotates" , "Imaginary central line" , "vertebra" , "Imaginary dividing line" , "all revolves around this" .)

I've seen this clue in The Times.
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