Notice how the graph and mapping diagram visualize the following facts that for the sine, cosine and tangent functions.

Theorem: TRIG.SHAPE The shape of the sine, cosine, and tangent functions.
Increasing and Decreasing:
The justification for these facts can be understood by considering the mapping diagrams for the unit circle definitions of these functions as well as the more common graphical view.
On the graphs of these functions, this is visualized by the "vertices" of the wave curves. On the mapping diagram this is visualized by the fact that the arrows all land above (and below) the values $-1 $  (and $1$) on the target axis.