Graph-2. Velocity versus time, when velocity is constant
Equation defining velocity as a function of time for this case is
and the graph is a straight line parallel to time axis, Fig.1
We know that displacement in linear motion with constant velocity is equal to the product of velocity and the time of duration of the motion. Therefore, the displacement is equal to the area below the blue line as explained in Fig.1.
Remember: the area under the line representing the velocity as a function of time is equal to displacement.
This will be very useful for analyzing graphs of motion with varying velocity.