var mx, my,
    X, Y;

function setup()
{
  createCanvas(360, 360); 
  X=width-1;
  Y=height-1;
}

function draw()
{
  background(100);  
  mx=mouseX;
  my=mouseY;

  strokeWeight(1);  
  stroke(0, 0, 255);    line( 0, my,  X, my);
                        line(mx,  0, mx,  Y);
  strokeWeight(2);  
  stroke(0, 255, 0);    line( 0,  0, mx, my);
                        line( X,  0, mx, my);
                        line( 0,  Y, mx, my);
                        line( X,  Y, mx, my);
}