"charts below plot the ratio of quotes to trade value (similar to Quotes/Trades but in way to better compare the shifting lower transaction size since 2006). Each line shows the one-minute average value for all NMS stocks (about 8,000) for each minute of one trading day (9:30 to 16:00 ET), color coded by age: older dates towards the violet end of the spectrum, while recent days are colored towards the red end. The thick red line is Feb 25, 2013..."
764,962,237,673 quotes & 43,688,052,096 trades were processed to create the next chart
3 Day, 15 Minute SPY Chart: