How to Add Confidence Interval to Ggplot2 in R

The ggplot2 library is used to plot different graphs. We can use the geom_smooth() or geom_ribbon() method to add confidence interval lines or bands to the plot.

This tutorial demonstrates adding a confidence interval to ggplot2 in R.

Add Confidence Interval to ggplot2 in R

First, we need to create the data frame on which we will plot the ggplot2.

Example code:

x <- 1:80
y <- rnorm(80) + x / 8
low <- y + rnorm(80, - 2, 0.1)
high <- y + rnorm(80, + 2, 0.1)
data_frame <- data.frame(x, y, low, high)
head(data_frame)

The code above creates an example data frame with 80 lines and four columns; the x and y are the values that will be plotted, and the values high and low are the confidence intervals for those values.

See output for the code, which is the head of the data frame:

  x          y         low     high
1 1  0.7819640 -1.29439898 2.719669
2 2  0.8893052 -1.09524369 2.885744
3 3  1.8762126  0.03061524 3.910596
4 4 -0.3713966 -2.34534103 1.683949
5 5  1.9099850 -0.04985367 4.026922
6 6  1.2793110 -0.82542732 3.199673

The next step is to plot the ggplot2. But first, we need to install and load the ggplot2 package:

install.packages("ggplot2")
library("ggplot2")

Once the package is loaded, now we can plot the ggplot:

gg_plot <- ggplot(data_frame, aes(x, y)) +
  geom_point()
gg_plot

The code above will plot the graph:

We can finally add the confidence interval to the ggplot2 using the geom_smooth() or geom_ribbon() methods.

Add confidence interval lines using the geom_smooth() method:

## Add confidence interval lines
gg_plot + geom_smooth(aes(ymin = low, ymax = high), alpha = 0.2)

Add confidence interval bands using the geom_ribbon() method:

Complete Example Code

Here is the complete example code used in this tutorial.

x <- 1:80
y <- rnorm(80) + x / 8
low <- y + rnorm(80, - 2, 0.1)
high <- y + rnorm(80, + 2, 0.1)
data_frame <- data.frame(x, y, low, high)
head(data_frame)

install.packages("ggplot2")
library("ggplot2")

gg_plot <- ggplot(data_frame, aes(x, y)) +
  geom_point()
gg_plot

## Add confidence interval lines
gg_plot + geom_smooth(aes(ymin = low, ymax = high), alpha = 0.2)

## Add confidence interval bands
gg_plot + geom_ribbon(aes(ymin = low, ymax = high), alpha = 0.2)