You can use **geom_smooth()** to add confidence interval lines to a plot in ggplot2:

**library(ggplot2)
some_ggplot +
geom_point() +
geom_smooth(method=lm)**

The following examples show how to use this syntax in practice with the built-in mtcars dataset in R.

**Example 1: Add Confidence Interval Lines in ggplot2**

The following code shows how to create a scatterplot in ggplot2 and add a line of best fit along with 95% confidence bands:

**library(ggplot2)
#create scatterplot with confidence bands
ggplot(data=mtcars, aes(x=mpg, y=wt)) +
geom_point() +
geom_smooth(method=lm)
**

The blue line represents the fitted linear regression line and the grey bands represent the 95% confidence interval bands.

**Example 2: Modify Level of Confidence Interval**

By default, **geom_smooth()** uses 95% confidence bands but you can use the **level** argument to specify a different confidence level.

For example, we may choose to create 90% confidence bands instead:

**library(ggplot2)
#create scatterplot with 90% confidence bands
ggplot(data=mtcars, aes(x=mpg, y=wt)) +
geom_point() +
geom_smooth(method=lm, level=0.90)
**

The smaller the confidence level you use, the more narrow the confidence interval bands will be around the regression line.

**Example 3: Modify Appearance of Confidence Interval Lines**

You can also use the **color** and **fill** arguments to modify the color of the regression line and the color of the confidence interval bands, respectively:

**library(ggplot2)
#create scatterplot with custom confidence interval lines
ggplot(data=mtcars, aes(x=mpg, y=wt)) +
geom_point() +
geom_smooth(method=lm, color='red', fill='lightblue')
**

The regression line is now red and the confidence interval bands are filled in with light blue.

**Note**: You can find the complete documentation for the **geom_smooth()** function here.

**Additional Resources**

The following tutorials explain how to perform other common operations in ggplot2:

How to Change the Legend Title in ggplot2

How to Rotate Axis Labels in ggplot2

How to Fix in R: could not find function “ggplot”