关于 EXT JS grid不显示header

下面的代码，是我照着大侠写过的EXT JS改的，为什么grid不显示header的内容？也就是grid的数据列不显示“节目名称”等列名？
<!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>
<meta content="text/html; charset=gb2312" http-equiv="Content-Type" />
<title></title>
<link rel="stylesheet" type="text/css" href="ext-2.2/resources/css/ext-all.css" />
</head>
<body style="padding:15px;">
<div id="grid_panel"></div>
<iframe name="hf" src="about:blank" style="display:none;"></iframe>
<script type="text/javascript" src="ext-2.2/adapter/ext/ext-base.js"></script>
<script type="text/javascript" src="ext-2.2/ext-all.js"></script>
<script type="text/javascript">
//<![CDATA[
var oForm, oGrid;
var aData = [
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"],
["大话西游","10:00","12:00","2:00","09/05/12","审核通过","刘三"]
];
var oProg = Ext.data.Record.create([{name:'prog_name'},{name:'start_time'},{name:'end_time'},{name:'during_time'},{name:'date'},{name:'state'},{name:'person'}]);
var oStore = new Ext.data.Store({
    proxy:new Ext.data.MemoryProxy(aData),
    reader:new Ext.data.ArrayReader({},oProg)
    });
oStore.load();function addHandler(){
    oGrid.stopEditing();
    var last_rd = oStore.getAt(oStore.getCount()-1);
    var rd = new oProg({prog_name:'',start_time:last_rd.get('end_time'),end_time:''});
    oStore.add(rd);
    oGrid.startEditing(oStore.getCount()-1, 0);
}
function delHandler(){
    var sm = oGrid.getSelectionModel();
    var rds = sm.getSelections();
    for(var i=0;i<rds.length;i++){
        oStore.remove(rds[i]);
    }
}
function PrintIt(){
    window.print()
}
function doMouseDown(oObj, iIndex){
    var oSM = oObj.getSelectionModel();
    oSM.clearSelections();
    oSM.selectRow(iIndex, true);
}
var oSM    = new Ext.grid.CheckboxSelectionModel();
oSM.handleMouseDown = doMouseDown;
var cm = new Ext.grid.ColumnModel([
    new Ext.grid.RowNumberer(),
    oSM,
    {header:"节目名称",dataIndex:'prog_name',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{
        focus:function(obj){
            var sm = oGrid.getSelectionModel();
            var rd = sm.getSelected();
            this.rdid = rd.id;
        },
        change :function(obj){
        var val = obj.getRawValue();
        var rd = oStore.getById(this.rdid);
        rd.set("prog_name", val);
        }}
    })},
    {header:"开始时间",dataIndex:'start_time',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{
        focus:function(obj){
            var sm = oGrid.getSelectionModel();
            var rd = sm.getSelected();
            this.rdid = rd.id;
        },
        change :function(obj){
        var val = obj.getRawValue();
        var rd = oStore.getById(this.rdid);
        rd.set("start_time", val);
        }}
    })},
    {header:"结束时间",dataIndex:'end_time',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{
        focus:function(obj){
            var sm = oGrid.getSelectionModel();
            var rd = sm.getSelected();
            this.rdid = rd.id;
        },
        change :function(obj){
        var val = obj.getRawValue();
        var rd = oStore.getById(this.rdid);
        rd.set("end_time", val);
        }}
    })},
    {header:"节目长度",dataIndex:'during_time',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{}
    })},
{header:"节目播出日期",dataIndex:'date',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{}
    })},
{header:"节目审核状态",dataIndex:'state',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{}
    })},
{header:"操作人",dataIndex:'person',width:80,editor:new Ext.form.TextField({allowBlank:false,listeners:{}
    })}
]);
function init(){
    oGrid    = new Ext.grid.EditorGridPanel({
    renderTo:"grid_panel",
    title:"节目表编排",
    monitorResize:true,
    autoHeight:true,autoWidth:true,
    sm:oSM,
    frame:true,width:600,
    clicksToEdit:1,  
store:oStore,
    tbar:[
    {xtype:'tbbutton',icon:'images/add.gif',text:'文件',cls:"x-btn-text-icon",handler:addHandler},
        {xtype:'tbbutton',icon:'images/add.gif',text:'添加节目',cls:"x-btn-text-icon",handler:addHandler},
        {xtype:'tbbutton',icon:'images/del.gif',text:'删除所选节目',cls:"x-btn-text-icon",handler:delHandler},
        {xtype:'tbbutton',icon:'images/del.gif',text:'打印输出',cls:"x-btn-text-icon",handler:PrintIt},
{xtype:'tbbutton',icon:'images/del.gif',text:'预览节目',cls:"x-btn-text-icon",handler:PrintIt},
        {xtype: 'tbspacer'}
        ],
    cm:cm//    autoExpandColumn:3,
    });
}window.onload    = function(){
    Ext.onReady(init);
}
//]]>
</script>
</body>
</html>