How will I draw a curve that represents the trajectory of a projectile?

Let me explain.

I want to experiment in Unity, and I like the idea of having a little, three-dimensional side-scrolling environment to screw around in. Because everything will be viewed from the side, even though it's 3D, the Z coordinate obviously isn't important when working with the physics in my "game".

Let's say that there's a character flinging a pebble from a slingshot. While the character aims, I want a line/curve to appear that represents the pebble's trajectory. (Pointing the mouse upwards will make the pebble reach higher, but travel a much lesser horizontal distance, if you know what I mean.)

It's like Angry Birds, how the line appears when you aim. For simplicity's sake, the line that is drawn in my "game", will just be basic. (A single pixel thick and coloured white, or something.) How is that drawn? Should I use particles? I can't imagine that it is GUI. So what do I use, and how do I use it.

This is a pretty big question, so I'm very sorry.