Numerical calculation of a negative acceleration

In figure i, what happens if you calculate the acceleration between t = 1.0 and 1.5 s?

Reading from the graph, it looks like the velocity is about -1 m/s at t = 1.0 s, and around -6 m/s at t = 1.5 s. The acceleration, figured between these two points, is

Even though the ball is speeding up, it has a negative acceleration.