How do you calculate displacement on a velocity vs. time graph?
Area under the curve - broken in two parts - the area above the and the area below the time axis. the sum (including a negative sign for area below) would give displacement.
Area below the curve is distance in a velocity time graph gives the distance, However, to find the displacement we need to break the area into two parts.
(i) the area between the curve and above the time axis and
(ii) the area between the curve and below the time axis
The area below the time axis would be negative as v is negative.
Adding the two gives the displacement.