Stocks opened with substantial losses, but have done some repair work as global equity markets extend last week’s sell-off. China’s Shanghai composite plunged 8.5% and erased gains for the year. The Nasdaq trimmed an 8.8% deficit to 5% while the S&P 500 pared a 5.3% loss to 4.1%. Meanwhile, the Dow Jones industrial average was off 642 points, or 4.5%, with all 30 issues in the red.