Question

I’m trying to create a line graph and keep getting an error when I add in the error bars (just getting started with R, so apologies!). I’m not sure why – help would be appreciated!

Group = c("a","a","b","b","a","a","b","b")
Time = c(1,2,1,2,1,2,1,2)
Code = c("A","A","A","A","B","B","B","B")
Mean = (2,6,7,5,6,1,2,8)
SE = c(1.9,1.7,1.5,1.3,2,1.8,2.3,1.5)
dataset=data.frame(Group,Time,Code,Mean,SE)

ggplot(data=dataset) + geom_line(aes(x=Time,y=Mean,colour=Code,linetype=Group))+ 
  scale_x_continuous(breaks=c(1,2)) + 
  scale_linetype_manual(values=c(2,1)) + 
  geom_point(aes(x=Time,y=Mean,colour=Code,linetype=Group)) + 
  geom_errorbar(aes(ymin=Mean-SE,ymax=Mean+SE),width=.1,position=dodge)

The problem has to do with the last line — code works fine without it. But with it, I get: Error in eval(expr, envir, enclos) : object 'x' not found.

So what am I doing wrong with the geom_errorbar line?

Answer 1

The first thing I would try is to define the aesthetics only once and do so in the ggplot() function. Ie.

ggplot(data=dataset,aes(x=Time,y=Mean,colour=Code,linetype=Group,ymin=Mean-SE,ymax=Mean+SE)) + 
geom_line() + 
scale_x_continuous(breaks=c(1,2)) + 
scale_linetype_manual(values=c(2,1)) + 
geom_point() + 
geom_errorbar(width=.1,position='dodge')

This is because ggplot doesn’t guarantee to pass all of the variables that are in the original dataset and weird results can result from depending on this.

Edit: I just noticed that x never gets defined for geom_errorbar, adding x=Time to either the aes of ggplot() or geom_errorbar() should fix the problem. However, doing the latter is really not recommended.

If you give example data (eg. dput) I would be able to help you further.