createCanvas(240, 120);

  strokeWeight(12);

}

​

function draw() {

  background(220);

  stroke(255);

  line(mouseX, 0, mouseX, height); // white line

  // var mx = mouseX/2 + 60;

  // example: mouseX at 200, mx at 160 (offsetting)

  var mx = map(mouseX, 0, width, 60, 180);

  stroke(0);

  line(mx, 0, mx, height); // black line

}