This multi-line chart uses an invisible Voronoi tessellation to handle mouseover; the closest point to the mouse on any line is highlighted. Click the checkbox in the top-right to toggle the visibility of the Voronoi overlay.
Note that because d3.geom.voronoi does not handle coincident points (and this data from the government comes pre-rounded to a tenth of a degree), d3.nest is used to collapse coincident points before constructing the Voronoi.