You want to graph a time series of prices using qplot in the ggplot2 program.

First create the date and price vectors.  The price vector will be the spread between two separate prices:

> date = data$date

> wcswti16 = data$wcs16 - data$wti16

Call the qplot, with the date vector as the x-axis and the spread as the y axis, “p”.  Then add an abline to p at the y-axis point of your choice:

> p = qplot(date,wcswti16)
> p + geom_abline(intercept = -15, slope = 0)

This is the result: