How to Fix: geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?

One error you may encounter when using R is:

geom_path: Each group consists of only one observation. Do you need to adjust
the group aesthetic?

This error usually occurs when you attempt to create a line chart using ggplot2 but the x-axis variable is a factor, which can cause issues when connecting the points in the plot.

The following example shows how to fix this error in practice.

How to Reproduce the Error

Suppose we have the following data frame in R that contains information about the sales of a certain product during various years:

#create data frame
df <- data.frame(year=factor(c(2017, 2018, 2019, 2020, 2021, 2022)),
                 sales=c(23, 30, 35, 41, 48, 44))

#view data frame
df

  year sales
1 2017    23
2 2018    30
3 2019    35
4 2020    41
5 2021    48
6 2022    44

Now suppose we attempt to create a line chart using ggplot2 to visualize the sales by year:

library(ggplot2)

#attempt to create line chart
ggplot(df, aes(year, sales)) +
  geom_point() +
  geom_line()

geom_path: Each group consists of only one observation. Do you need to adjust
the group aesthetic?

A scatterplot is produced instead of a line plot because the x-axis variable (year) is a factor.

We also receive the geom_path error message.

How to Fix the Error

The easiest way to fix this error is to specify group=1 within the aes() function:

library(ggplot2)

#create line chart
ggplot(df, aes(year, sales, group=1)) +
  geom_point() +
  geom_line()

Notice that a line chart is created and no error message appears.

The reason that group=1 fixes this error is because line graphs require the data points to be grouped so that ggplot2 knows which points to connect.

In this scenario, we want all of the points in the plot to be connected so we specify group=1.