var frame = 0;

function setup() {
  frameRate(3);
}

function draw() {
  createCanvas(400, 400);
  background(220);

  var i = 0;

  var x1 = 0;
  var y1 = 0;
  var x2 = 400;
  var y2 = 0;
  var y_step = 400 / 50;

  while (i < 50) {
    if (i == frame) {
      stroke("red");
      line(x1, y1, x2, y2 + y_step * frame);
    } else {
      stroke("black");
      line(x1, y1, x2, y2 + y_step * i);
    }

    i += 1;
  }

  frame += 1;
  if (frame > 50) {
    frame = 0;
  }
}