Sales of plug-in electric cars in the U.S. had a low-to-average month in September, with the sole bright spot being the astounding sales of the BMW i3 electric hatchback. Two new 2016 models, the entirely redesigned Chevy Volt and the longer-range 107-mile Nissan Leaf, should start this month, but last month's sales likely included selldowns of their predecessors as buyers waited for the new and more desirable versions.