19

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.

JPD
  • 2,561
  • 5
  • 22
  • 26

2 Answers2

28

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))

enter image description here

Andrie
  • 176,377
  • 47
  • 447
  • 496
  • Ah ok, I didn't know the formula had to be replaced with it. Thanks for solving this. – JPD Jun 06 '12 at 09:59
  • Note that I didn't "replace" the formula. The formula wasn't necessary, so I removed it. – Andrie Jun 06 '12 at 11:41
  • 16
    Pet peeve: "the error message is self explanatory". If it was self explanatory OP wouldn't have asked the question to begin with. – Twitch May 17 '14 at 05:40
1

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)
tim
  • 3,559
  • 1
  • 33
  • 46