For the tooltips, the hover over are isn't very user friendly.

For example, using the closest point on the X-axis in a chart rich in data points is hard to get the exact point of interest. Usually these points occur where there is a distinguishing feature which Voronoi diagrams excel in. Such as a spike in a line diagram.

Even when there is no feature to focus on, the Voronoi diagram will have effectvily the same interaction as the current method.