This scatterplot is step 1 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 the actual circles, no Voronoi is attached in the starting example. This is used to illustrate the fact that it’s actually rather hard to mouse over each of the small circles. In the next examples, an invisible Voronoi grid is used to trigger the event that improves UX. You can see the final result here
You can find all of the steps here