While 2017 saw higher U.S. sales of plug-in electric cars than any previous year, expectations for 2018 sales are running even higher. The Chevrolet Bolt EV will be available in all 50 states for a full year, the new 2018 Nissan Leaf with a 151-mile range is now on sale, and of course the Tesla Model 3 will be a factor. How much of a factor…