================================== Thanks SPY includes volume,your slippage, real trading-, investing. SPX chart has no volume on it
If you do not see volume on the SPX charts it mean that your chart provider does not offer it. S&P 500 (SPX) volume is the summary volume of the 500 stocks listed in the S&P 500. SPX also has Advance Decline Volume (not just NYSE), it is also has momentum advance decline volume, it is also has Selling and buying volume and much more... Just see the chart below chart courtesy of MarketVolume