Question

How do you calculate displacement on a velocity vs.time graph?

Answer 1

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.
Explanation:
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.
$\left(i\right)$ the area between the curve and above the time axis and
$\left(ii\right)$ 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.