Vega.on event handler

.on("mouseover", function(event, item) {
  // invoke hover properties on cousin one hop forward in scenegraph
  view.update({
    props: "hover",
   items: item.cousin(1)
  });
});

      

        
        
        
      

    

I got this code from this link:

https://github.com/trifacta/vega/blob/master/examples/interact/index.html

I didn't understand what is

items: item.cousin(1);

      

        
        
        
      

    

I use it for plotting on vega.