Stocks fight back as oil pinballs back above $30

By Marc Jones LONDON (Reuters) - Markets went on another rollercoaster ride on Tuesday as hopes that big oil producers may start tightening supply drove crude oil prices higher and pulled stocks out of a worldwide dive that included a six-percent plunge in China. European shares were flat and Wall Street was expected to start firmer with the focus on a flurry of data and the first Federal Reserve meeting this year. London , Frankfurt and Paris had initially dropped as much as 1.8 percent after t