Despite the growing appeal of the Tesla Models S and X, and excitement about a coming 200-mile-range Chevy Bolt, plug-in electric vehicles still account for a scant fraction of U.S. auto sales. Though March marked the biggest electric vehicle sales month in history, according to industry publication InsideEVs, those sales accounted for far less than 1% of all new vehicle sales - if you don't count plug-in hybrids that also have gasoline engines under the hood.