Handling Datatables' standard footer:{content:"summColumn"} functionality

Hi Team ,

I have designed a page with datatable having custom check box in its first column and a footer which does the total of “fee” column.

How can i change the value of footer total : i.e basically i want to add/substract the total with the fee column value on-click of the respective custom checkbox.

Please find the complete code below.

Counter in the footer
	<link rel="stylesheet" type="text/css" href="../common/samples.css">
	<style type="text/css">
		.checked{
			color:green;
			font-weight: bold; cursor:pointer;
		}
		.notchecked{
			color:red;
			font-weight:bold; cursor: pointer;
		}
	</style>
</head>
<body>
	<div class='header_comment'>Counter in the foo1ter</div>
	<div class='sample_comment'>Type any text to filter the grid.</div>
	<div id="testA"></div>
	<script type="text/javascript" charset="utf-8">
	
	var small_film_set = [
		{ id:1, doctorName:"Piyush C", date:"20/04/2014", fee:200},
		{ id:2, doctorName:"Meera mehra", date:"22/04/2014", fee:250},
		{ id:3, doctorName:"Naidu B", date:"24/04/2014", fee:300},
		{ id:4, doctorName:"Roseiz", date:"01/05/2014", fee:100},
		{ id:5, doctorName:"Antony", date:"20/06/2014", fee:100},
		{ id:6, doctorName:"Lehar M K", date:"23/06/2014", fee:500}
	];
	webix.ready(function(){
		grid = new webix.ui({
			container:"testA",
			view:"datatable",
			columns:[
				{ id:"rank",	header:"Pay?", template:custom_checkbox, css:"rank",  width:50,	sort:"int",
					footer:{text:"Total:", colspan:2}
				},
				{ id:"doctorName",	header:["Dr. Name"], width:200 },
				{ id:"date",	header:"Date" , width:80,	sort:"int"},
				{ id:"fee",	header:"Fee", 	width:100,	sort:"int",
					footer:{ content:"summColumn" }
				}
			],
			autoheight:true,
			autowidth:true,
			data:small_film_set,
			footer:true,								
			checkboxRefresh:true,
		});			
	
				function custom_checkbox(obj, common, value){
					if (value){
						//alert(obj.doctorName);
							return "<div class='webix_table_checkbox checked'> YES </div>";
						}
					else{
							//alert(obj.doctorName);
							return "<div class='webix_table_checkbox notchecked'> NO </div>";
						}
				};
	});		
	</script>
</body>

You can define a custom counter with any kind of logic.
Please check the next example http://webix.com/snippet/7e1bfb0a