Line in bar chart (3)
I believe the answer is:
arc
'line' is the definition.
(I know that line can be written as arc)
'in bar chart' is the wordplay.
'in' says the answer is hidden in the clue.
'ARC' can be found hidden inside 'bar chart'.
(Other definitions for arc that I've seen before include "Segment of circumference" , "section of Circle Line" , "Bow" , "Section of circumference" , "part of turning circle?" .)