关于highcharts 实时曲线的问题，谢谢！

我利用jquery 的highcharts图表实现实时曲线，现在有一个问题不知如何解决，
http://www.highcharts.com/demo/dynamic-update/grid 参靠地址 代码如下：<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WellScan.aspx.cs" Inherits="sinoperm.WellPOC.UI.UI.WellScan" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script type="text/javascript" src="../Scripts/jquery/jquery-1.4.1.js"></script>
    <script type="text/javascript" src="../Scripts/highcharts.js"></script>
    <script type='text/javascript'>
        $(function () {
            $(document).ready(function () {                Highcharts.setOptions({                    global: {                        useUTC: false                    }                });                var chart;                chart = new Highcharts.Chart({                    chart: {                        renderTo: 'container',                        type: 'spline',                        marginRight: 10,                        events: {                            load: function () {                                // set up the updating of the chart each second                                var series = this.series[0];                                setInterval(function () {                                    var x = (new Date()).getTime(), // current time                                y = Math.random();                                    series.addPoint([x, y], true, true);                                }, 1000);                            }                        }                    },                    title: {                        text: 'Live random data'                    },                    xAxis: {                        type: 'datetime',                        tickPixelInterval: 150                    },                    yAxis: {                        title: {                            text: 'Value'                        },                        plotLines: [{                            value: 0,                            width: 1,                            color: '#808080'                        }]                    },                    tooltip: {                        formatter: function () {                            return '<b>' + this.series.name + '</b><br/>' +                        Highcharts.dateFormat('%Y-%m-%d %H:%M:%S', this.x) + '<br/>' +                        Highcharts.numberFormat(this.y, 2);                        }                    },                    legend: {                        enabled: false                    },                    exporting: {                        enabled: false                    },                    series: [{                        name: 'Random data',                        data: (function () {                            // generate an array of random data                            var data = [],                        time = (new Date()).getTime(),                        i;                            for (i = -19; i <= 0; i++) {                                data.push({                                    x: time + i * 1000,                                    y: Math.random()                                });                            }                            return data;                        })()                    }]                });            });
        });
       </script>
</head>
<body>
    <form id="form1" runat="server">
    
        <div id="container" style="height: 400px">
        </div>
   
    </form>
</body>
</html>
 事件中产生的随机点没有任何问题，现在我要把这个Y轴的点通过后台json中获取数据进行改变，我通过ajax对这个代码进行了更改，数据获取到后，页面的曲线应该是随着时间的移动向左消失的，但加了ajax后不消失而是不段在页面中累加变挤，不知为什么，是不是有什么属性需要设置还是我的程序写的有问题，还是只能在event事件进行操作呢。谢谢