Animating Arrow to Mimic Drawing

I feel like I'm missing something very basic here, and am hoping someone doesn't mind pointing me in the right direction.  I want to animate an arrow shape so that it appears to be extending... as if someone is drawing it.

For example, say you have a baseball diamond on the screen.  There it a pitcher character on the mound and a batter at home plate.  I want to show the path the ball is going to take when thrown... as an arrow that begins at the pitcher and extends down towards the batter.

I feel like I must be overlooking something in Animations or Motion Paths to accomplish this.  Any assistance is appreciated.