public void OnPointerEnter(PointerEventData eventData)
        {
            graph.tooltip.Show();
            object obj = node.GetInputValue <object>(port.fieldName, null);

            if (obj != null)
            {
                graph.tooltip.label.text = obj.ToString();
            }
            else
            {
                graph.tooltip.label.text = "n/a";
            }
        }