Defining ports in json response

I want to define the no of ports, port id of each port, port location of individual ports in the node data array or a json response ,I used dynamic port concept in this but always the port is in centre of the node only , how to achieve this

      new go.Binding("itemArray", "bottomPort"),
          $(go.Shape, "circle",new go.Binding("portId","portId"),
          { fill: "red",
            stroke: null,
            desiredSize: new go.Size(5,5),
            alignment: go.Spot.Bottom, 
            alignmentFocus: go.Spot.Bottom, 
            fromSpot: go.Spot.Bottom, toSpot: go.Spot.Bottom,  
            cursor: "pointer",fromLinkable:true,toLinkable:true