Como obter o mouse relativo do mouse clique no elemento
document.getElementById('clickme').onclick = function clickEvent(e) {
// e = Mouse click event.
var rect = e.target.getBoundingClientRect();
var x = e.clientX - rect.left; //x position within the element.
var y = e.clientY - rect.top; //y position within the element.
console.log("Left? : " + x + " ; Top? : " + y + ".");
}
Graceful Gnat