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)\")
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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))

Answer 2

FYI, this error can also be encountered (and fixed) using the simple qplot interface to ggplot2

The error message is not explanatory enough for a few people at least :-). In this case, the key is to include only the contents of the suggested aesthetic

library(ggplot2)
qplot(factor(Sepal.Length), Sepal.Width, geom = c("smooth"), data= iris)
# error, needs aes(group=1)
qplot(factor(Sepal.Length), Sepal.Width, geom = c("smooth"), group = 1, data= iris)