How to add a line in ggplot2 using geom_path in a bar graph

Question

I wrote this code:

df = data.frame(a=c('A','A','B','B'),b=c('X','Y','X','Y'),c=c(1,2,3,2))

q = ggplot(df, aes(a, y=c,fill=b))+
  geom_bar(position=position_dodge(), stat="identity")

path = data.frame(x=c(1,2),y=c(2,2))
q = q + geom_path(data=path,aes(x=x, y=y))

I want to get a plot that have a horizontal line that start at the junction of the first two bar and end at the junction of the last two bar at height(position in Y axis)2.

However,my code give me an error:

"Error in eval(expr, envir, enclos) : object 'b' not found"

update: Thanks to Chris, moving the "fill" to geom_bar solves the orignal problem. But now, a new problem happens. I changed my code to:

    library(ggplot2)
df=data.frame(a=c('A','A','B','B'),b=c('X','Y','X','Y'),c=c(1,2,3,2),err=c(.1,.2,.1,.2))
q = ggplot(df, aes(a, y=c))+
  geom_bar(aes(fill=b),position=position_dodge(), stat="identity")+
  geom_errorbar(aes(ymin=c-err,ymax=c+err), width=0.3, lwd = 1, position=position_dodge(0.9))

path = data.frame(x=c(1,2),y=c(2,2))
q = q + geom_path(data=path,aes(x=x, y=y))

print(q)

Because the "fill" is not in the original ggplot() function, the position of the error bar is messed up.

Chris Watson Chris Watson · Accepted Answer · 2015-07-13T04:51:08

You should move the aes in your first ggplot call:

q <- ggplot(df, aes(a, y=c)) +
    geom_bar(aes(fill=b), position=position_dodge(), stat='identity')

path <- data.frame(x=c(1,2),y=c(2,2))
q <- q + geom_path(data=path,aes(x=x, y=y))

EDIT: added code for completeness

How to add a line in ggplot2 using geom_path in a bar graph

1 Answers