jQuery EasyUI的datagrid数据不能在页面显示

Question

使用ThinkPHP框架，在index.html中显示一个grid，但是数据显示不上去，只能显示表头。不知道怎么回事，路径都没什么问题的。
index.html页面的javascript代码如下。
<script language="javascript">
$(function(){
    $('#tt').datagrid({
        url: 'datagrid_data2.json',
        title: 'DataGrid - ContextMenu',
        width: 700,
        height: 'auto',
        fitColumns: true,
        columns:[[
            {field:'itemid',title:'Item ID',width:80},
            {field:'productid',title:'Product ID',width:120},
            {field:'listprice',title:'List Price',width:80,align:'right'},
            {field:'unitcost',title:'Unit Cost',width:80,align:'right'},
            {field:'attr1',title:'Attribute',width:250},
            {field:'status',title:'Status',width:60,align:'center'}
        ]],
    });
});
</script>body部分就是一个table，<table id="tt"></table>
json代码如下：
{"total":28,"rows":[
    {"productid":"FI-SW-01","unitcost":10.00,"status":"P","listprice":36.50,"attr1":"Large","itemid":"EST-1"},
    {"productid":"K9-DL-01","unitcost":12.00,"status":"P","listprice":18.50,"attr1":"Spotted Adult Female","itemid":"EST-10"},
    {"productid":"RP-SN-01","unitcost":12.00,"status":"P","listprice":28.50,"attr1":"Venomless","itemid":"EST-11"},
    {"productid":"RP-SN-01","unitcost":12.00,"status":"P","listprice":26.50,"attr1":"Rattleless","itemid":"EST-12"},
    {"productid":"RP-LI-02","unitcost":12.00,"status":"P","listprice":35.50,"attr1":"Green Adult","itemid":"EST-13"},
    {"productid":"FL-DSH-01","unitcost":12.00,"status":"P","listprice":158.50,"attr1":"Tailless","itemid":"EST-14"},
    {"productid":"FL-DSH-01","unitcost":12.00,"status":"P","listprice":83.50,"attr1":"With tail","itemid":"EST-15"},
    {"productid":"FL-DLH-02","unitcost":12.00,"status":"P","listprice":63.50,"attr1":"Adult Female","itemid":"EST-16"},
    {"productid":"FL-DLH-02","unitcost":12.00,"status":"P","listprice":89.50,"attr1":"Adult Male","itemid":"EST-17"},
    {"productid":"AV-CB-01","unitcost":92.00,"status":"P","listprice":63.50,"attr1":"Adult Male","itemid":"EST-18"}
]}详细代码见附件:
http://bbs.thinkphp.cn/forum.php?mod=viewthread&tid=48763