Cadillac said Sunday, April 19, 2015 at the Shanghai auto show that it will offer a plug-in hybrid electric vehicle variant of the big CT6 flagship sedan. CT6 goes on sale in the U.S. in December. Caddy gave no    timetable for rolling out the PHEV version in China, the U.S. and other countries. The CT6 is shown here being unveiled at the New York auto show in March.