This scatterplot is step 2 of my blog on Using a D3 Voronoi grid to improve a chart’s interactive experience in which the tooltip and the event are both triggered by an invisible Voronoi grid that lies over the scatterplot. This is used to illustrate the fact that you still need one more step, and that is to attach the tooltip to the circle that represents the seed of each Voronoi cell, which you can see in the next example
You can find all of the steps here