function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(255);

  points.push([mouseX, mouseY]);

  if(points.length > 100){

    points.shift();

  }

  for(var i = 0; i < points.length - 1; i ++){

    line(points[i][0], points[i][1], points[i + 1][0], points[i + 1][1])

  }

}