After four years of steady growth, U.S. plug-in electric car sales were essentially flat last year. Low gas prices and increased demand for SUVs had a dampening effect on the segment, which still accounts for a very small fraction of U.S. new-car sales. However, in other parts of the world, it was a different story. Recommended: Think you’re a car buff? Take the quiz. Global electric-car sales actually increased significantly last year, thanks to strong…