You need to get the page x and y coordinates, and then minus the canvas offset to get them relative to the canvas.

function q(event) {

    event = event || window.event;



    var canvas = document.getElementById('canvas'),

        x = event.pageX - canvas.offsetLeft,

        y = event.pageY - canvas.offsetTop;



    alert(x + ' ' + y);

}

jsFiddle.

You should consider attaching events unobtrusively, i.e. not using the onclick HTML attribute.