jqGrid sum of column values from selected rows only in footer

HTML

<link rel="stylesheet" href="http://www.trirand.com/blog/jqgrid/themes/redmond/jquery-ui-custom.css">
<link rel="stylesheet" href="http://www.trirand.com/blog/jqgrid/themes/ui.jqgrid.css">
<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>
<table id="grid"></table>
<div id="pager"></div>

JavaScript

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

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",tax:"21.00",total:"320.00"},
        {id:"9",invdate:"2007-09-01",name:"test3",note:"note3",tax:"30.00",total:"430.00"},
        {id:"11",invdate:"2007-10-01",name:"test",note:"note",tax:"10.00",total:"210.00"},
        {id:"12",invdate:"2007-10-02",name:"test2",note:"note2",tax:"20.00",total:"320.00"},
        {id:"13",invdate:"2007-09-01",name:"test3",note:"note3",tax:"30.00",total:"430.00"},
        {id:"14",invdate:"2007-10-04",name:"test",note:"note",tax:"10.00",total:"210.00"},
        {id:"15",invdate:"2007-10-05",name:"test2",note:"note2",tax:"20.00",total:"320.00"},
        {id:"16",invdate:"2007-09-06",name:"test3",note:"note3",tax:"30.00",total:"430.00"},
        {id:"17",invdate:"2007-10-04",name:"test",note:"note",tax:"10.00",total:"210.00"},
        {id:"18",invdate:"2007-10-03",name:"test2",note:"note2",tax:"20.00",total:"320.00"},
        {id:"19",invdate:"2007-09-01",name:"test3",note:"note3",tax:"30.00",total:"430.00"},
        {id:"21",invdate:"2007-10-01",name:"test",note:"note",tax:"10.00",total:"210.00"},
        {id:"22",invdate:"2007-10-02",name:"test2",note:"note2",tax:"20.00",total:"320.00"},
       ...