let centerY = 100;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  centerX = centerX + 1;

  console.log(centerX);

  line(0, 0, centerX, centerY);

  line(400, 0, centerX, centerY);

  line(400, 400, centerX, centerY);

  line(0, 400, centerX, centerY);

}