Customize Multi Series Tooltips

I am using this code to customize a tooltip on a multiline xy scatter chart. How would I modify the tooltip so each series would have its own unique label, nameArr, nameArr1, nameArr2...?

tooltip: {
format: {
title: function(x) {
var indOfVal = engagmentArr.indexOf(x);
return nameArr[indOfVal - 1]
},
name:function(){
return engagmentArr[0];
}
},
},