Plot line? (4)
I believe the answer is:
axis
'plot line?' 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 "Alliance of powers eg Germany, Italy and Japan in WWII" , "WW2 Nazi alliance" , "vertebra" , "Line of symmetry" , "Imaginary centre of rotation" .)