The “geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?” error often occurs when our variable is a factor variable. Check out our tutorial below to find the best solution.
What causes the “geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?” error?
Reproduce the “geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?” error
Example 1:
Let’s say we will have this data set that contains information about the number of passengers taking underground railway systems per year in six major cities.
dataFrame <- data.frame(city=c("London", "Paris", "Tokyo", "Washington DC", "Kyoto", "Los Angeles"),
number_of_passengers=c(775, 1191, 1928, 144, 45, 50))
dataFrameOutput:
city number_of_passengers
1 London 775
2 Paris 1191
3 Tokyo 1928
4 Washington DC 144
5 Kyoto 45
6 Los Angeles 50
Now we will draw a line plot with ggplot2 to represent the numbers of passengers taking underground railway systems in the six cities. First, we have to install and load the ggplot2 package:
install.packages("ggplot2")
library(ggplot2)Then, we will create the line plot with these codes:
ggplot(dataFrame, aes(city, number_of_passengers)) +
geom_point() +
geom_line() +
ggtitle("Number of passengers taking underground railway systems per year (in millions)") +
theme(plot.title = element_text(size = 12, face = "bold", color = "blue"))Output:
As you can see, instead of a line plot like we want, we get a scatterplot. Alongside, we also so get an error message:
geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?This is because, in order to form a line graph, R must know what are the points to connect.
How to solve this problem?
Solution 1: Group the data points
Because R needs to know which points to connect, we have to group the data points. Therefore, we can specify group=1 in the aes()
Fix code for example 1:
dataFrame <- data.frame(city=c("London", "Paris", "Tokyo", "Washington DC", "Kyoto", "Los Angeles"),
number_of_passengers=c(775, 1191, 1928, 144, 45, 50))
dataFrame
library(ggplot2)
ggplot(dataFrame, aes(city, number_of_passengers, group=1)) +
geom_point() +
geom_line() +
ggtitle("Number of passengers taking underground railway systems per year (in millions)") +
theme(plot.title = element_text(size = 12, face = "bold", color = "blue"))Output:
Solution 2: Use numeric x-variable
Let’s take a look at example 2:
We will draw a line plot for this data set giving information about the number of factories in England and Wales from 1851 to 1901.
dataFrame <- data.frame(year=as.numeric(c("1851", "1861", "1871", "1881", "1891", "1901")),
number_of_factories=c(225, 227, 622, 721, 625, 600))
dataFrame
library(ggplot2)
ggplot(dataFrame, aes(as.numeric(year), number_of_factories)) +
geom_point() +
geom_line() +
ggtitle("Number of factories in England and Wales from 1851 to 1901") +
theme(plot.title = element_text(size = 12, face = "bold", color = "green"))Output:
In this example, we can try to convert the year into numeric using as.numeric(). And since the x-variable is numeric, we don’t have to use group=1.
Summary
“geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?” is common error in R. It often occurs when R does not know which points to connect. The two simple solutions are to group the data points using group=1 or coerce the value to numeric.
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