translate and scale to set origin at center and flip y-axis

traditional_cartesian.pde

void setup() {
  size(400, 400);
}

void draw() {
  background(0);
  translate(width/2, height/2); 
  scale(1, -1);

  stroke(255);
  line(-200, 0, 200, 0);
  line(0, -200, 0, 200);


  // Draw point 100,50
  fill(255);
  ellipse(100, 50, 16, 16);

  // Draw point -100,-150
  fill(255);
  ellipse(-100, -150, 16, 16);


}