Cut-off and Cook’s distance plot with abline

General
1

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)
summary(fit)

qqPlot(fit,labels=row.names(stack.loss), id.method= "identify",
simulate=TRUE, main="QQ Plot")

outlierTest(fit)

fitted(fit)["21"]
fitted(fit) ["4"]

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.

2

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