Example:

In short, translate to the pivot point, then do your rotation, and use coordinates relative to the pivot point (the line starts at 0, 0, for example).
In this little sample, the push/popMatrix() instructions are not necessary (they are done automatically around draw()), but can be useful if you have several separate rotations.
Example, based on the above code:

1. float angle = 0.0;
2. 
   
3. void setup() {
4.   size(220, 220);
5.   smooth();
6. }
7. 
   
8. void draw() {
9.   angle = map(abs(-2500 + millis() % 5000), 0, 2500, -1, 1);
10.   background(#005522);
11. 
    
12.   pushMatrix();
13.   translate(110, 20);
14.   rotate(angle + HALF_PI);
15.   stroke(255);
16.   strokeWeight(3);
17.   line(0, 0, 50, 0);
18.   noStroke();
19.   fill(128);
20.   ellipse(70, 0, 40, 40);
21.   popMatrix();
22.  
23.   pushMatrix();
24.   translate(170, 150);
25.   rotate(angle / 2 - HALF_PI);
26.   stroke(#000088);
27.   strokeWeight(5);
28.   line(0, 0, 50, 0);
29.   popMatrix();
30. }