Until recently it had been the mission of the S&P 500 index to eliminate gaps in the calculation of their index.  After hours changes in stock prices were assessed as those stocks opened, but the opening price of the index was usually equal to the closing price the day before.  In other words, an multi-day intra-day price graph would be continuous and any gaps in the components would be reflected as a sharp decline during trading hours.

Below we highlight the S&P 500, DJIA, and Nasdaq indices, and their day-by-day gaps since the start of the 2002 bull market.