I have custom technical studies I designed expressly for spread trading. My clients use my studies along with eSignal, CQG, or Bloomberg charting packages - depending upon what they are comfortable with.
We use statistical methods like correlation and cointegration studies to design and build synthetic spread combinations. Our hedge ratios use volatility and currency adjustments to take as much delta directionality as practical out of the trade.
The volatility and daily trading range of a particular spread dictates it's holding timeframes. Big difference between a Eurodollar Condor and an RBOB Crack Spread.
Some of my clients use the intermarket correlations to arbitrage between products utilizing 'lead-lag' strategies and skip the spread stuff altogether. They are spread traders who never get around to closing the legs, essentially. That is what I was trying to illustrate with the ES Correlators chart, and the Copper vs. Rio Tinto chart.
There is alot of multi-dimensionality and flexibility to "spread" trading. Arbitrage, lead-lag, fully hedged multiple leg positions designed for fairly long holding periods, market making, etc. etc.