A graph that shows acceleration plotted against time for a particle moving in a straight line. Let v(t) and a(t) be the velocity and acceleration, respectively, of the particle at time t. The acceleration-time graph is the graph y = a(t), where the t-axis is horizontal and the y-axis is vertical with the positive direction upwards. With the convention that any area below the horizontal axis is negative, the area under the graph between t = t1 and t = t2 is equal to v(t2)−v(t1).