You have a matrix like this:

You want to only look at Q1 prices for the spread between two products.  You can subset the price vectors by the qtr vector like this:

gcq116 = data[data$qtr==1,c("gc87q116")]

rbobq116 = data[data$qtr==1,c("rbobq116")]

Create the date vector:

> dateqtr1 = data[data$qtr==1,c("date")]

You now have two price vectors showing only the Q1 prices.  Graph the spread between the two vectors like this:

> qplot(dateqtr1,gcq116 - rbobq116)

which returns this: