SPY trades off ES yes. ES trades off Globex session, current day, the day before, last week.
I have SPY and QQQ charts that I rarely look at.
ES/NQ/RTY and Tick Symbols, Up/Down Vol, Adv/Decl's of all three are all I need.
Well ok also $VIX, $DXY, Treasuries and Currencies, Gold and Crude. lol