Observe from the graph, The first 5 minutes someone was filling up the bathtub, when the height of the water was increases linearly.

At the 5 minute mark, the person got into the tub. The person was lying in the bathtub for about seven minutes. The V shape indicates that the person was drained some of the water and refilled the tub again.

At 18 minutes the person got out of the tub, which causes the height of the water level was decreases steadily. It took nearly four minutes to reach the height of the water level is 0.