var polyClicked = function(event) {
  var x, y = null;
  
  if (event.layerX && event.layerY) {
    x = event.layerX;
    y = event.layerY;
  } else if (event.offsetX && event.offsetY) {
    x = event.offsetX;
    y = event.offsetY;
  }
 
  alert("clicked at x: " + x + ", y: " + y);
};