Another day, another $1 billion down the market’s drain. Spare a thought for Warren Buffett, whose portfolio is not doing him any favors this week. On Monday Buffett lost nearly $1 billion on his third-largest investment, IBM, after the company posted disappointing earnings. On Tuesday, Coca-Cola did the same thing, posting third-quarter revenue that fell short of expectations and warning of currency headwinds. Coke is Buffett’s second-largest investment and has been one of the stalwarts of his portfolio for decades.