var y = 0;

​

function setup() {

  createCanvas(window.innerWidth, window.innerHeight);

}

​

function draw() {

  //this trail fading effect is achieved by setting opacity to low value

  background(220,0,0,10);

  //remove circle stroke

  noStroke();

  //use mousex and mousey for circle pos

  x = mouseX;

  y = mouseY;

  //draw circle

  circle(x,y,50);

}