ggplot2 error bars : Quick start guide - R software and data visualization


This tutorial describes how to create a graph with error bars using R software and ggplot2 package. There are different types of error bars which can be created using the functions below :

  • geom_errorbar()
  • geom_linerange()
  • geom_pointrange()
  • geom_crossbar()
  • geom_errorbarh()

Add error bars to a bar and line plots

Prepare the data

ToothGrowth data is used. It describes the effect of Vitamin C on tooth growth in Guinea pigs. Three dose levels of Vitamin C (0.5, 1, and 2 mg) with each of two delivery methods [orange juice (OJ) or ascorbic acid (VC)] are used :

library(ggplot2)
df <- ToothGrowth
df$dose <- as.factor(df$dose)
head(df)
##    len supp dose
## 1  4.2   VC  0.5
## 2 11.5   VC  0.5
## 3  7.3   VC  0.5
## 4  5.8   VC  0.5
## 5  6.4   VC  0.5
## 6 10.0   VC  0.5
  • len : Tooth length
  • dose : Dose in milligrams (0.5, 1, 2)
  • supp : Supplement type (VC or OJ)

In the example below, we’ll plot the mean value of Tooth length in each group. The standard deviation is used to draw the error bars on the graph.

First, the helper function below will be used to calculate the mean and the standard deviation, for the variable of interest, in each group :

#+++++++++++++++++++++++++
# Function to calculate the mean and the standard deviation
  # for each group
#+++++++++++++++++++++++++
# data : a data frame
# varname : the name of a column containing the variable
  #to be summariezed
# groupnames : vector of column names to be used as
  # grouping variables
data_summary <- function(data, varname, groupnames){
  require(plyr)
  summary_func <- function(x, col){
    c(mean = mean(x[[col]], na.rm=TRUE),
      sd = sd(x[[col]], na.rm=TRUE))
  }
  data_sum<-ddply(data, groupnames, .fun=summary_func,
                  varname)
  data_sum <- rename(data_sum, c("mean" = varname))
 return(data_sum)
}

Summarize the data :

df2 <- data_summary(ToothGrowth, varname="len", 
                    groupnames=c("supp", "dose"))
# Convert dose to a factor variable
df2$dose=as.factor(df2$dose)
head(df2)
##   supp dose   len       sd
## 1   OJ  0.5 13.23 4.459709
## 2   OJ    1 22.70 3.910953
## 3   OJ    2 26.06 2.655058
## 4   VC  0.5  7.98 2.746634
## 5   VC    1 16.77 2.515309
## 6   VC    2 26.14 4.797731

Barplot with error bars

The function geom_errorbar() can be used to produce the error bars :

library(ggplot2)
# Default bar plot
p<- ggplot(df2, aes(x=dose, y=len, fill=supp)) + 
  geom_bar(stat="identity", color="black", 
           position=position_dodge()) +
  geom_errorbar(aes(ymin=len-sd, ymax=len+sd), width=.2,
                 position=position_dodge(.9)) 
print(p)
# Finished bar plot
p+labs(title="Tooth length per dose", x="Dose (mg)", y = "Length")+
   theme_classic() +
   scale_fill_manual(values=c('#999999','#E69F00'))

Note that, you can chose to keep only the upper error bars

# Keep only upper error bars
 ggplot(df2, aes(x=dose, y=len, fill=supp)) + 
  geom_bar(stat="identity", color="black", position=position_dodge()) +
  geom_errorbar(aes(ymin=len, ymax=len+sd), width=.2,
                 position=position_dodge(.9)) 

Read more on ggplot2 bar graphs : ggplot2 bar graphs

Line plot with error bars

# Default line plot
p<- ggplot(df2, aes(x=dose, y=len, group=supp, color=supp)) + 
  geom_line() +
  geom_point()+
  geom_errorbar(aes(ymin=len-sd, ymax=len+sd), width=.2,
                 position=position_dodge(0.05))
print(p)
# Finished line plot
p+labs(title="Tooth length per dose", x="Dose (mg)", y = "Length")+
   theme_classic() +
   scale_color_manual(values=c('#999999','#E69F00'))

You can also use the functions geom_pointrange() or geom_linerange() instead of using geom_errorbar()

# Use geom_pointrange
ggplot(df2, aes(x=dose, y=len, group=supp, color=supp)) + 
geom_pointrange(aes(ymin=len-sd, ymax=len+sd))
# Use geom_line()+geom_pointrange()
ggplot(df2, aes(x=dose, y=len, group=supp, color=supp)) + 
  geom_line()+
  geom_pointrange(aes(ymin=len-sd, ymax=len+sd))

Read more on ggplot2 line plots : ggplot2 line plots

Dot plot with mean point and error bars

The functions geom_dotplot() and stat_summary() are used :

The mean +/- SD can be added as a crossbar , a error bar or a pointrange :

p <- ggplot(df, aes(x=dose, y=len)) + 
    geom_dotplot(binaxis='y', stackdir='center')
# use geom_crossbar()
p + stat_summary(fun.data="mean_sdl", fun.args = list(mult=1), 
                 geom="crossbar", width=0.5)
# Use geom_errorbar()
p + stat_summary(fun.data=mean_sdl, fun.args = list(mult=1), 
        geom="errorbar", color="red", width=0.2) +
  stat_summary(fun.y=mean, geom="point", color="red")
   
# Use geom_pointrange()
p + stat_summary(fun.data=mean_sdl, fun.args = list(mult=1), 
                 geom="pointrange", color="red")

Read more on ggplot2 dot plots : ggplot2 dot plot

Infos

This analysis has been performed using R software (ver. 3.2.4) and ggplot2 (ver. 2.1.0)


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