EasyUi DataGrid 绑定数据格式问题

如果显示汇总记录则需设置页脚属性：
首先设置showFooter:true,

然后后台计算出合计数据，一起传过来，类似如下：
{"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"}
],"footer":[
{"unitcost":19.80,"listprice":60.40,"productid":"Average:"},
{"unitcost":198.00,"listprice":604.00,"productid":"Total:"}
]}

注意：“footer”中的数据是一个与主表数据格式一样的表格，字段与之对应，我的处理方式是新建一个Datatable，然后传到页面中。

DataTable footer = new DataTable();
footer.Columns.Add("unitcost", typeof(string));

DataRow drfooter = footer.NewRow();

drfooter["unitcost"] = "合计：" + sum;

footer.Rows.Add(drfooter);

需要汇总某一列，在新表中只加那一列即可。