Why is this code not functioning with IE 8?

Could anyone help me to figure out why this code does not work on IE 8 ? (It works on Chrome, Firefox,开发者_StackOverflow中文版 Opera).

The code use Raphäel.js library, the code allow user to mouse drag Raphäel.js elements (e.g. circle, rectanglar)

Please have a look here:

var paper = Raphael(0, 0, '100%', '100%');

var circle = paper.circle(75, 75, 50);
var rect = paper.rect(150, 150, 50, 50);

var set = paper.set();

set.push(circle, rect);
set.attr({
    fill: 'red',
    stroke: 0
});

var ox = 0;
var oy = 0;
var dragging = false;

set.mousedown(function(event) {
    ox = event.screenX;
    oy = event.screenY;
    set.attr({
        opacity: .5
    });
    dragging = true;
});

set.mousemove(function(event) {
    if (dragging) {
        set.translate(event.screenX - ox, event.screenY - oy);
        ox = event.screenX;
        oy = event.screenY;
    }
});

set.mouseup(function(event) {
    dragging = false;
    set.attr({
        opacity: 1
    });
});

there seems to be an error in the way ie8 and ie9 handle mouse move events http://css.dzone.com/news/internet-explorer-8-fix-event-