Chart Arithmetic
You can add or subtract the bid/ask average prices for two or more contracts, and chart the sum as a single value.
To chart combined contracts
From the trading window click the Charts icon.
Select the Combined Contracts radio button at the top of the New Chart window.
In the contracts table, define multiple contracts.
Use the Ratio field to subtract a value, by putting a minus sign in front of the ratio.
If desired, enter an offset amount.
Choose the Time Period and How to Show values.
Click OK.