The U.S. stock market swung wildly on Monday, with by far the biggest single-day drop in the history of the Dow Jones industrial average — at one stage down 1,597 points from Friday’s close. The Dow finished down 4.6 percent, at 24,345.75, wiping out its gains for the year. By percentage, however, it was more like a 5.5 earthquake than a Loma Prieta.