This example demonstrates mouseenter and mouseleave events. Mouseenter and mouseleave events are triggered only when the mouse enters or leaves the parent container; unlike mouseover and mouseout, they are not triggered when the mouse moves between descendant elements. This makes it easier to listen for when the mouse enters a group of elements and avoid unnecessary flickering.
For browsers that do not yet support mouseenter and mouseleave events, D3 provides a polyfill on top of the mouseover and mouseout events.