The S&P 500 (^GSPC) just broke out to new highs. The next question is whether it holds. Thursday’s Chart of the Day looked at the index’s rare 10% surge in 11 trading days. The setup is historically bullish, but this version comes with a catch: The S&P 500 is already back near the highs. That makes the next confirmation test less about speed and more about participation. The best tell may be breadth. One way to track that is the advance-decline line, a running measure of how many S&P 500 stocks are ri ...