html5-canvas怎麼設置鼠標事件？怎麼獲取裡面的點？

canvas怎麼設置鼠標事件？怎麼獲取裡面的點？

function() {

var graph = document.getElementById('canvas');

if (graph.getContext){
var g = graph.getContext('2d');
g.strokeStyle = '#000';
g.beginPath();
for (var i = 0, l = a.length; i < l; i++) {
g.arc((i + 1) * 100,a[i],3,0,Math.PI*2,true);
g.fill();
g.lineTo((i + 1) * 100,a[i]);

}
g.fillStyle="black";
g.stroke();
g.closePath();

想做出鼠標滑動到點上時，有一個信息框顯示出來，大神們幫我！

最佳回答：

用這個吧，忘記計算canvas的位置了

 <style>
    #note{position:absolute;display:none;border:solid 1px #000;background:#fff}
</style>
<canvas id="canvas" style="border:solid 1px #000;margin:100px 0 0  100px;" width="1000"></canvas>
<div id="note"></div>
<script>
    var a = [10, 20, 30, 40, 50, 20, 30, 40],note=document.getElementById('note');
    var c = document.getElementById('canvas');
    function getLeftRight(o) {
        var p = { x: o.offsetLeft, y: o.offsetTop };
        while (o = o.offsetParent) { p.x += o.offsetLeft; p.y += o.offsetTop; }
        return p;
    }
    function hitCheck(x, y, point, r) {//檢查是否移動到點上，直接按照正方形檢查了，圓形比較麻煩
        var x1 = point.x, y1 = point.y;
        return x1 + r > x && x1 - r < x && y1 + r > y && y1 - r < y;
    }
    if (c.getContext) {
        var g = c.getContext('2d');
        var points = [];//原點圓心集合
        var r = 3;//半徑
        var cP = getLeftRight(c);//獲取canvas對象位置
        for (var i = 0, l = a.length; i < l; i++) {
            points[i] = { x: (i + 1) * 100, y: a[i] };
            g.beginPath();
            g.moveTo(i * 100, i - 1 < 0 ? 0 : a[i - 1]);
            g.lineTo((i + 1) * 100, a[i]);
            g.arc((i + 1) * 100, a[i], r, 0, Math.PI * 2, true);
            g.fill();
            g.stroke();
            g.closePath()
        }
        var timer;
        c.addEventListener('mousemove', function (e) {
            clearTimeout(timer);
            var x = e.clientX, y = e.clientY;
            timer = setTimeout(function () {
                for (var i = 0; i < points.length; i++) 
                    if (hitCheck(x - cP.x, y - cP.y, points[i], r)) {
                        note.style.display = 'block';
                        note.style.left = points[i].x +cP.x+ r + 1 + 'px';
                        note.style.top = points[i].y + cP.y + r + 1 + 'px';
                        note.innerHTML = 'X：' + points[i].x + '　　Y：' + points[i].y;
                        return
                    }

                note.style.display = 'none';
            }, 100);
        }, false);
    }
</script>