jqGrid dynamic drag-n-drop grouping

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.23/themes/redmond/jquery-ui.css">
<link rel="stylesheet" href="http://www.trirand.com/blog/jqgrid/themes/ui.jqgrid.css">
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.23/jquery-ui.min.js"></script>
<script src="http://www.trirand.com/blog/jqgrid/js/i18n/grid.locale-en.js"></script>
<script src="http://www.trirand.com/blog/jqgrid/js/jquery.jqGrid.js"></script>
<div id="groups">
    <h1 class="ui-widget-header">Grouping</h1>
    <div class="ui-widget-content">
        <ol>
            <li class="placeholder">Drop headers here</li>
        </ol>
    </div>
</div><table id="grid"></table>
<div id="pager"></div>

CSS

#groups li { margin: 0 3px 3px 3px; padding: 0.4em; padding-left: 1.5em; height: 18px; }
#groups li span { position: absolute; margin-left: -1.3em; }

JavaScript

$.jgrid.no_legacy_api = true;
$.jgrid.useJSON = true;

var $grid= $("#grid");
var mydata = [
        {id:"1",invdate:"2010-05-24",name:"test",note:"note",tax:"10.00",total:"2111.00"} ,
        {id:"2",invdate:"2010-05-25",name:"test2",note:"note2",tax:"20.00",total:"320.00"},
        {id:"3",invdate:"2007-09-01",name:"test3",note:"note3",tax:"30.00",total:"430.00"},
        {id:"4",invdate:"2007-10-04",name:"test",note:"note",tax:"10.00",total:"210.00"},
        {id:"5",invdate:"2007-10-05",name:"test2",note:"note2",tax:"20.00",total:"320.00"},
        {id:"6",invdate:"2007-09-06",name:"test3",note:"note3",tax:"30.00",total:"430.00"},
        {id:"7",invdate:"2007-10-04",name:"test",note:"note",tax:"10.00",total:"210.00"},
        {id:"8",invdate:"2007-10-03",name:"test2",note:"note2",amount:"300.00",tax:"21.00",total:"320.00"},
        {id:"9",invdate:"2007-09-01",name:"test3",note:"note3",amount:"400.00",tax:"30.00",total:"430.00"},
        {id:"11",invdate:"2007-10-01",name:"test",note:"note",amount:"200.00",tax:"10.00",total:"210.00"},
        {id:"12",invdate:"2007-10-02",name:"test2",note:"note2",amount:"300.00",tax:"20.00",total:"320.00"},
        {id:"13",invdate:"2007-09-01",name:"test3",note:"note3",amount:"400.00",tax:"30.00",total:"430.00"},
        {id:"14",invdate:"2007-10-04",name:"test",note:"note",amount:"200.00",tax:"10.00",total:"210.00"},
        {id:"15",invdate:"2007-10-05",name:"test2",note:"note2",amount:"300.00",tax:"20.00",total:"320.00"},
        {id:"16",invdate:"2007-09-06",name:"test3",note:"note3",amount:"400.00",tax:"30.00",total:"430.00"},
        {id:"17",invdate:"2007-10-04",name:"test",note:"note",amount:"200.00",tax:"10.00",total:"210.00"},
        {id:"18",invdate:"2007-10-03",name:"test2",note:"note2",amount:"300.00",tax:"20.00",total:"320.00"},
        {id:"19",invdate:"2007-09-01",name:"test3",note:"note3",amount:"400.00",tax:"30.00",total:"430.00"},
       ...