Complex sentence: Cumulative global sales of highway legal plug-in electric passenger cars and light utility vehicles passed the 1.5 million unit milestone in May 2016, of which, about a third were sold in 2015.
Split into simpler sentences: Cumulative global sales of highway legal plug-in electric passenger cars and light utility vehicles passed the 1.5 million unit milestone in May 2016. Of these, about a third were sold in 2015.