EMERGING MARKETS-Brazil markets swing on S&P warning; real touches 3.43/USD

SAO PAULO, July 28 (Reuters) - Brazil's financial markets swung widely on Tuesday after Standard & Poor's warned that the country could lose its coveted investment-grade rating in the coming year. Brazil's currency, the real started the day lower against the dollar, crossing the 3.40 level for the first time in over 12 years. S&P in the early afternoon affirmed Brazil's credit rating of BBB-minus, the agency's lowest investment grade rating, and revised the outlook on that rating to neg