You can use the following basic syntax to draw a trend line on a plot in ggplot2:

ggplot(df, aes(x=xvar, y=yvar)) + geom_point() + geom_smooth(method=lm) #add linear trend line

The following examples show how to use this syntax in practice with the following data frame:

#create data frame df <- data.frame(x=c(1, 2, 3, 3, 5, 7, 9), y=c(8, 14, 18, 25, 29, 33, 25)) #view data frame df x y 1 1 8 2 2 14 3 3 18 4 3 25 5 5 29 6 7 33 7 9 25

**Example 1: Add Linear Trend Line**

The following code shows how to add a linear trend line to a scatterplot in ggplot2:

library(ggplot2) ggplot(df, aes(x=x, y=x)) + geom_point() + geom_smooth(method=lm) #add linear trend line

**Example 2: Add Linear Trend Line & Specify Confidence Region**

We can use the **level** argument to specify the confidence level to use for the shaded confidence region in the plot:

library(ggplot2) ggplot(df, aes(x=x, y=x)) + geom_point() + geom_smooth(method=lm, level=0.99)

Note that the default confidence level is 0.95. By specifying a confidence level of 0.99, our shaded confidence region on the plot became even wider.

**Example 3: Add Linear Trend Line & No Confidence Region**

We can use the **se=FALSE** argument to hide the shaded confidence region around the trend line:

library(ggplot2) ggplot(df, aes(x=x, y=x)) + geom_point() + geom_smooth(method=lm, se=FALSE, col='red', size=2)

**Example 4: Add Curved Trend Line**

If we don’t specify a method to use for **geom_smooth()**, a curved loess line will be used by default:

library(ggplot2) ggplot(df, aes(x=x, y=x)) + geom_point() + geom_smooth()

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

**Additional Resources**

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

How to Adjust Line Thickness in ggplot2

How to Set Axis Limits in ggplot2

How to Remove Gridlines in ggplot2

How to Change Background Color in ggplot2