The Electric Drive Transportation Association has been tracking U.S. Sales of plug-in electric cars since 2010, when the first Nissan LEAF and Chevrolet Volt mass-market cars were introduced:
https://electricdrive.org/index.php?ht=d/sp/i/20952/pid/20952
President Barack Obama during his 2011 State-of-the-Union address challenged the nation to put one million plug-in electric cars on U.S. roads during his term.
It took a little longer, but the benchmark was achieved during October 2018, when 9,465 plug-in hybrid electric vehicles and 24,638 battery-electric vehicles were sold.
Total sales since 2010 were estimated at 1,015,980 at the end of October 2018.
NEVA blog posted by Stan Hanel, Outreach Coordinator