Creating Error Bars in R (ggplot2) - r

Question

I've been working on creating a bar graph with error bars to depict group differences for a dataset that I have. But the error bars are coming out funky, in that they are appearing further above the bar and in the middle of a bar.
My code:
ggplot(MRS_Hippo_NAA_Cre_Data_copy, aes(Type, Hippo_6_9NAACre, fill=Type)) +
geom_bar(stat="summary", fun.y="mean", colour="black", size=.3) +
geom_errorbar(aes(ymin=meanNAA-NAAse, ymax=meanNAA+NAAse), width=.2,
position=position_dodge(.9)) + labs(x="Group", y="Right Posterior NAA/Cre") +
scale_fill_manual(values=c("#0072B2", "#D55E00"), name="Group") + theme(text =
element_text(size=18))`
This produced this graph:
I calculated the standard error by using the following function:
std <- function(x) sd(x)/sqrt(length(x))
x=Hippo_6_9NAACre
Not sure why the graph is producing funky error bars. Can anyone help or provide insight?

Answer 1

I had very recently a similar problem.
To solve it, first of all you may want to remove the layer
geom_errorbar(aes(ymin=meanNAA-NAAse,
ymax=meanNAA+NAAse), width=.2, position=position_dodge(.9))
and rather use a layer with the statsummary function again. That will generate the error bars separated for group.
As you want the bars indicating the standard error, you must create an appropriate function that returns the needed values, such that can be used from statsummary.
Find below a working example with iris dataset.
library(ggplot2)
## create a function for standard error that can be used with stat_summary
# I created the function inspecting the results returned by 'mean_cl_normal' that is the
# function used in some examples of stat_summary (see ?stat_summary).
mean_se = function(x){
se = function(x){sd(x)/sqrt(length(x))}
data.frame(y=mean(x), ymin=mean(x)+se(x), ymax=mean(x)-se(x))
}
## create the plot
p = ggplot(iris, aes(x = Species, y = Sepal.Length), stat="identity") +
stat_summary(fun.y = mean, geom = "col", fill = "White", colour = "Black", width=0.5) +
stat_summary(fun.data = mean_se, geom = "errorbar", width=0.2, size=1)
# print the plot
print(p)