Generating a graph is something that all R enthusiasts must challenge at one point or another. While you are doing so, you will surely encounter this error message: “geom_path: each group consists of only one observation. do you need to adjust the group aesthetic?”

For the less experienced, this code is among the hardest to figure out. This article will make it more manageable for you.

What is “geom_path: each group consists of only one observation. do you need to adjust the group aesthetic?”?

The “geom_path: each group consists of only one observation. do you need to adjust the group aesthetic?” error message is a unique ggplot2 library error. It will appear if you try to use the ggplot function with only character strings. However, it does not tell you exactly what is wrong.

This issue makes this error one of the most frustrating to work through for many inexperienced programmers.  It may be as frustrating as the undefined object error.

After all, its main cause lies in the ggplot function’s inability to graph with character strings. This weakness is well-hidden and can only be found if you read through the function’s instruction notes carefully. 

In other words, you won’t find any problem troubleshooting all group arguments, as your function is still producing the graph. It’s just not plotting anything on this graph. If you don’t already know about it before, it may cost you hours of fumbling through the codes for a non-existent answer.

Example

Here is a small code snippet containing a data frame built on a pond’s water level.

df <- data.frame(y=c("97", "99", "01", "03", "05", "07", "09"), 

water_lvl = c(99, 104, 100, 98, 104, 99, 110)) 

With this data frame, we can try to make a line plot with the ggplot2 function and illustrate the fluctuation of the pond’s water level between 1997 and 2009 with the code below:

library(ggplot2) 

ggplot(df, aes(y, water_lvl)) + 

geom_point() + 

geom_line() + 

ggtitle("Pond water level between 1997 and 2009") + 

theme(plot.title = element_text(size = 8, face = "bold"))

When you run the code, a graph will still appear, but there is no line, and the error message will pop up.

There are two approaches to solving this error.

Solution 1

This solution is more of a temporary patch on a wound. To be more specific, it will allow the line to form, but it won’t take care of the underlying cause. All you need to do is pass in group = 1 to ses(). The code will turn into:

library(ggplot2) 

ggplot(df, aes(y, water_lvl, group = 1)) + 

geom_point() + 

geom_line() + 

ggtitle("Pond water level between 1997 and 2009") + 

theme(plot.title = element_text(size = 8, face = "bold"))

Essentially, we are telling R which data points it should connect by grouping them together. As we want to group all the points, we assign the value 1 to group.

When you run the code again, you will see that the error is no longer there.

Solution 2

The second approach takes care of the core issue by assigning the input numeric values. You can do so by utilizing as.numeric(). After using it, you can use the original code to test out.

df <- data.frame(y=as.numeric(c("97", "99", "01", "03", "05", "07", "09")), 

water_lvl = c(99, 104, 100, 98, 104, 99, 110)) 

library(ggplot2) 

ggplot(df, aes(y, water_lvl)) + 

geom_point() + 

geom_line() + 

ggtitle("Pond water level between 1997 and 2009") + 

theme(plot.title = element_text(size = 8, face = "bold"))

Conclusion

“geom_path: each group consists of only one observation. Do you need to adjust the group aesthetic?” can appear intimidating due to its length. However, you will soon realize that it’s actually quite easy to take care of once you understand its true nature.

In our eyes, this problem is as easy to fix as it is to make. Once you read through this article, it shouldn’t cause you any problems anymore.