createCanvas(400, 400);

} 

​

function draw() { 

  background(220);

    noFill();

    stroke(0, 0, 240);

    strokeWeight(20);

    ellipse(200, 200, 300, 200);

    stroke(255);

    strokeWeight(10);

    arc(200, 200, 300, 200, PI * 2,

            map(mouseX, 0, width, 0, TWO_PI), CHORD);

}