X or Y, on a graph (4)
I believe the answer is:
axis
'x or y on a graph' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for axis that I've seen before include "x or y line on a graph" , "Spindle - German/Italian alliance - deer" , "Fixed reference line" , "all revolves around this" , "Feasible" .)