Graph for Vertical Velocity of an airplane

I'm a fresh Calculus $$1$$ student struggling to make this graph somehow, I've been studying Chapter $$1$$ from James Stewart's single variable Calculus early transcendentals. Excercise $$1.1$$ is easy up until this question:

An airplane takes off from an airport and land an hour later at another airport, $$400$$ miles away. If $$t$$ represents the time in minutes since the plane has left the terminal building, let $$x(t)$$ be the horizontal distance traveled and $$y(t)$$ be the altitude of the plane: Sketch a possible graph of the vertical velocity.

This site draws the graph but I can't understand this, why is this so? Why the graph flips? I'm terrible when physics comes in Math, any formula, I guess $$v_0\sin\theta - gt$$, would be helpful, in terms of explaining the graph. If this graph is correct, I'm looking for an explanation of the sketch.