How to get total number of rows & average of votes

Hi All:
For footer:{text:“Total:”, colspan:3} … I like to display the total number of rows? how? where to make calculations?
For footer:{ content:“summColumn” } … instead of sum, how can I get the average?

		grid = new webix.ui({
			container:"testA",
			view:"datatable",
			columns:[
				{ id:"rank",	header:"", css:"rank",  		width:50,	sort:"int",
					footer:{text:"Total:", colspan:3}
				},
				{ id:"title",	header:["Film title",{ content:"textFilter"}], width:200 },
				{ id:"year",	header:"Released" , width:80,	sort:"int"},
				{ id:"votes",	header:"Votes", 	width:100,	sort:"int",
					footer:{ content:"summColumn" }
				}
			],
			autoheight:true,
			autowidth:true,
			data:small_film_set,
			footer:true
		});

you can introduce your own math function
http://webix.com/snippet/8ac606bb