Using the Faraway package and the stackloss data. I cannot put in an abline when plotting the influential points in my data using the cutoff and Cooks distance code below:
fit <- lm(stack.loss ~ ., data=stackloss)
qqPlot(fit,labels=row.names(stack.loss), id.method= "identify",
simulate=TRUE, main="QQ Plot")
cutoff <- 4/(nrow(stack.loss)-length(fit$coefficients)-2)
plot(fit, which=4, cook.levels=cutoff)
abline(h=cutoff, lty=2, col="red")
Everything in this code works except for the abline. Any assistance is greatly appreciated in this matter.