This scatterplot is part of the extension of my blog on Using a D3 Voronoi grid to improve a chart’s interactive experience. After writing that blog Franck Lebeau came with another version which uses large circles to define the tooltip region. I thought this was a great idea! But I made this variation on his code, because I felt that the extra code used in this example (versus the previous version 4) is more in line with the rest of the code. In version 6 you can find the same as this block, but with all of the blue lines and circles hidden or removed.
The tooltip now reacts when you hover over the blue circular regions and the tooltip is attached to the circle in the center
You can find all of the steps here