You can use the points() function to add points to an existing plot in R.

This function uses the following basic syntax:

points(df2$x, df2$y, col='red')

This particular syntax adds red points to an existing scatter plot in R using the variables called x and y from a data frame called df2.

The following example shows how to use this syntax in practice.

Example: Add Points to an Existing Plot in R

Suppose we use the plot() function to create the following scatter plot in R:

#create data frame
df1 <- data.frame(x=c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10),
                  y=c(4, 5, 5, 4, 6, 8, 12, 15, 19, 22)) 

#create scatterplot
plot(df1$x, df1$y, col='blue', pch=16)

Note: The col argument specifies the color of the points in the plot and the pch argument specifies the symbol to use. A value of 16 represents a filled-in circle.

Now suppose that we would like to add points from another data frame to the plot.

We can use the points() function to do so:

#create second data frame
df2 <- data.frame(x=c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10),
                  y=c(14, 12, 9, 9, 8, 5, 4, 5, 3, 2)) 

#add points from df2 to the existing scatter plot
points(df2$x, df2$y, col='red', pch=16)

Notice that the points from the second data frame have been added to the existing plot and are represented by a red color.

If we’d like, we can also use the legend() function to add a legend to the plot so that we can distinguish which points came from which data frame:

#add legend to plot
legend(x=1, y=22, legend=c('df1', 'df2'), fill=c('blue', 'red'))

Note: You can use the points() function as many times as you’d like to add points from as many data frames as you’d like to an existing plot.