JavaScript实现手动画线

在网上搜索了一些JavaScript画线的代码，综合了几个例子，作了一些修改和整合，实现了画线的功能，但是只是很简单的功能，只能实现单击画线，双击结束，由于本人初学JS，还不知道怎么实现撤消，及清除。页面代码如下：

<head>

var Working=false;

var points = [];

var lastPoint = {x:0,y:0};

var line = null;

document.ondblclick=function (){

if(!Working) return;

points = [];

lastPoint = {x:0,y:0}

Working=false;

}

document.onclick=function (){

if(!Working){

Working=true;

}

var s='<v:line from="'+event.x+','+event.y+'" to="'+event.x+','+event.y+'" style="position:absolute;left:0px;top:0px;"></v:line>';

lastPoint.x = event.x;

lastPoint.y = event.y;

points.push( {x:event.x,y:event.y} );

document.getElementById("show").innerHTML = "X:"+ event.x + " Y:" + event.y;

document.getElementById("show").style.display="";

var o=document.createElement(s);

document.body.insertAdjacentElement('BeforeEnd',o);

line = o;

}

document.onmousemove=function (){

if(!Working) return;

line.to = event.x + "," + event.y;

document.getElementById("dshow").innerHTML = "X:"+ event.x + " Y:" + event.y;

document.getElementById("dshow").style.display="";

}

</script>

v\:* {behavior:url(#default#VML);}

</style>

</head>

<body>

</div>

</div>

</body>

</html>