It is increasing in the intervals because the line is going up, and the numbers are increasing. It is increasing in the interval [0,3] and the interval [13,16]. It is decreasing in the interval [3,13] because the line on the graph is going down, and the values are decreasing.
The graph is misleading because it does not start out at 0 inches of rainfall. To make it accurate, you'd redraw it and start with 0 inches of rainfall and go up from there.
