Small jQuery plugin for center of parent or window! (Tweaked to set margin instead of position)

jquery.center.js

jQuery.fn.center = function(parent) {
    if (parent) {
        parent = $(this).parent();
    } else {
        parent = window;
    }
    $(this).css({
        "margin-top": ((($(parent).height() - $(this).outerHeight()) / 2) + $(parent).scrollTop() + "px"),
        "margin-left": ((($(parent).width() - $(this).outerWidth()) / 2) + $(parent).scrollLeft() + "px")
    });
return $(this);
}