Complex sentence: China, together with the U.S., had the world's largest country stock of plug-in electric passenger cars until September 2016, with the Chinese plug-in stock representing 29.2% of the global stock of highway legal plug-in electric passenger cars.
Split into simpler sentences: China, together with the U.S., had the world's largest country stock of plug-in electric passenger cars until September 2016. The Chinese plug-in stock represented 29.2% of the global stock of highway legal plug-in electric passenger cars.