r - Adding a simple lm trend line to a ggplot boxplot

Question

When adding a linear model trend line to a boxplot using standard R graphics I use:

boxplot(iris[,2]~iris[,1],col="LightBlue",main="Quartile1 (Rare)")
modelQ1<-lm(iris[,2]~iris[,1])
abline(modelQ1,lwd=2)

However, when using this in ggplot2:

a <- ggplot(iris,aes(factor(iris[,1]),iris[,2]))
a + geom_boxplot() +
geom_smooth(method = "lm", se=FALSE, color="black", formula=iris[,2]~iris[,1])

I get the following error:

geom_smooth: Only one unique x value each group.Maybe you want aes(group = 1)?

And the line does not appear on my plot.

The models used in both of these scenarios are identical. If anyone could point out where I'm going wrong, that would be great.

EDIT: Used the iris dataset as an example.

Answer 1

The error message is pretty much self-explanatory: Add aes(group=1) to geom_smooth:

ggplot(iris, aes(factor(Sepal.Length), Sepal.Width)) +
  geom_boxplot() +
  geom_smooth(method = "lm", se=FALSE, color="black", aes(group=1))