physics v-t graph

The motion consists of two stages. The first stage is the upward motion from the springboard to the highest point. The second stage is from the highest point to the water.

Eqaution of motion in the first stage: v = -4 + gt
where g is the acceleration due to gravity, equals to 10 m/s2
hence, v = -4 + 10t
The time when the highest point is reached = 4/10 s = 0.4 s

The graph for this motion is a straight line below the x-axis, starting at v = -4 m/s at t = 0 (i.e. coordinates (0, -4) ), and end with v = 0 m/s at t = 0 (i.e. coordinates (0.4, 0) ). The slope of the line is 10 m/s2

Eqaution of motion in the second stage: v = 10t
This is just a straight line that continues with the line drawn in the first stage, with the same slope.