U.S. stock index futures pointed to a sharply lower open on Tuesday, with Dow futures briefly falling more than 400 points, following some weaker-than-expected Chinese data. Two sets of key Chinese data disappointed traders on Tuesday. The official manufacturing purchasing managers' index (PMI) edged down to 49.7 in August from 50 in July, while the final Caixin/Markit manufacturing PMI came in at 47.3 in August, the lowest reading since March 2009. Even more worrying, China's services sector , which has been one of the lone bright spots in the sputtering economy, also showed signs of cooling, a similar business survey...