For the S&P futures, just plot the $PREM symbol. It's the premium between the cash and futures.Quote from kowboy:
Is there software available that plots and shows the difference between the futures and the cash market in real time?
As far as which one leads the other - neither one. That is, neither one always leads the other. You have a complex arbitrage going on betwen the cash/equities, the related ETF, and the futures - e.g., for the S&P you have a constantly changing dynamic between the SPX (i.e., the value of the underlying stocks that compose the S&P500), the SPY (the ETF that tracks the SPX), and the S&P futures. You also have some pit vs. globex arb going on too.
So at any given moment, the action of one of the three can produce reactions in the other two. Which one that's leading the action in the related securities changes throughout the day. A good idea is to watch the relative support/resistance on the connected symbols - a lot of times one of them will be hitting a key intraday resistance or support level while the others aren't yet but the others will end up stalling anyway.
