In http://forum.processing.org/topic/does-any-one-no-how-to-draw-a-dotted-line-with-from-particular-orgin-to-cureent-location-of-mousex-and-mousey thread, I shown how to draw dotted lines, which isn't interesting for you.
The point of interest is that for the demo I drawn two (dotted) lines anchored, one on the top, the other on the bottom, and following the mouseX position to join at the middle. Indeed, it uses atan2 for that.
I suggest to study how I did that and reuse the logic from there.

To remove some fluff from the code I pointed at, you have:

1. void setup()
2. {
3.   size(600, 600);
4.   smooth();
5. }
6. void draw()
7. {
8.   background(255);
9.   float y = height;
10.   float x = width / 2;
11.   float d = dist(x, y, mouseX, mouseY);
12.  
13.   float a = PI - atan2(y - mouseY, x - mouseX);
14.   float dx = height / 2 * (1 + cos(a));
15.   float dy = height / 2;
16.  
17.   line(x, y, dx, dy);
18. }

That's only a starting point, you still have to remove dependency on y, add angle constraints, etc.

rotate( radians( map( mouseX, 0, width, -60, 60 ) ) ); //?

you could always use the peasy cam lib...

http://mrfeinberg.com/peasycam/

Thanks for the replies guys! That was really really helpful!