One of my favorite error messages when you’re generating a graph, this error message occurs because you’ve fed the graph two different types of variable for the axes: one axis is a character string and one a numeric variable. The graph function only sees the numeric variable and thus views your data as a single observation point. Which isn’t a pretty picture….
The solution is to look at the data types for the series you are using to define the axes and making them consistent.
Naturally, in true R fashion, they hide this in complicated jargon rather than getting right to the point.
Description of the geom_path error
The “geom_path: each group consists of only one observation. do you need to adjust the group aesthetic?” error message occurs when using the ggplot function from the ggplot2 library. It is a result of giving it an axis variable that is a character string rather than a numeric value. This function needs numeric or date values on the axis to figure out the scale of the graph and plot the location of the data points. Unfortunately, this error message does not describe what is actually going on. This error message occurs because one axis is a character string and one a numeric variable, the function only sees the numeric variable and therefore only one observation point.
Explanation of the geom_path error
This error message occurs when you try to use character strings as an axis to the ggplot function. The problem results from the fact that the function cannot graph using character strings. The problem is not in the group argument because the function still produces the graph, but it does not plot anything giving you the error message. This is because the cause is in the geom path function. You can get the error when making a scatterplot using the geom point function. You also can get this error when making a line plot using the geom line function. In this case, it does not matter if you are plotting a single line or multiple lines.
> library(ggplot2)
> a = data.frame(a=c(“a”, “b”,”c”,”d”,”e”),
+ b=c(1, 3, 2, 8, 5))
> ggplot(a, aes(x=a, y=b)) + geom_line()
geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?
This is the simplest possible example. It is simply trying to graph letters and numbers. This is what results in the error message.
> library(ggplot2)
> a = data.frame(Day=c(“2021-11-22”, “2021-11-23″,”2021-11-24″,”2021-11-25″,”2021-11-26″,”2021-11-27”),
+ weight=c(139,138,137.5,137,141,140))
> ggplot(a, aes(x=Day, y=weight)) + geom_line()
geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?
This example is trying to plot dates with numeric values. Normally this would work but the dates are defined as character strings, not date values and this is what causes the error message. This error is most likely to occur when graphing dates.
How to fix the geom_path error
Here are examples fixing the two earlier examples of this error message.
> library(ggplot2)
> a = data.frame(a=c(1, 2, 3, 4, 5),
+ b=c(1, 3, 2, 8, 5))
> ggplot(a, aes(x=a, y=b)) + geom_line()
In this example, we simply change the letters in column “a” to numbers.
> library(ggplot2)
> a = data.frame(Day=c(“2021-11-22”, “2021-11-23″,”2021-11-24″,”2021-11-25″,”2021-11-26″,”2021-11-27”),
+ weight=c(139,138,137.5,137,141,140))
> a$Day = as.POSIXct(a$Day)
> ggplot(a, aes(x=Day, y=weight)) + geom_line()
In this example, we fix the error by setting the date strings equal to actual date values using the as.POSIXct function.
Despite the length of this error message, it is an easy one to fix once you understand the problem. It is an easy mistake to make, but also easy to fix. Once you learn how to fix it, it will be easy to avoid.