Canvas scatterplot w/ quadtree for point picking on hover, per: https://twitter.com/mbostock/status/733317794419212288
If you don’t want the full Voronoi effect because your chart has some big dead zones, you could add a distance threshold and only trigger the highlighting when the nearest point is within that distance.
If you need behavior that more closely mimics mouse/touch because your shapes have significant dimensions or their layer order matters, you could try colorpicking instead.