Drive a particle along a line and watch its position, velocity and acceleration graphs build in sync. The slope of the x–t graph is the velocity, and the shaded area under the v–t graph is the displacement.
Try this: set a = 0 → the v–t graph is flat and x–t is a straight ramp. Now make a negative with a positive u → the particle slows, stops at the v–t zero-crossing, then reverses and heads back.