Table of Contents
The ggproto objects cannot be combined together since they are not native R objects. Instead, ggplot2 provides several functions to manipulate ggproto objects, such as ggplot_add, ggplot_gtable, and ggplot_build, which can be used to combine them in a meaningful way. Additionally, it is recommended to use the ggplot() function to create ggproto objects in the first place.
One error you may encounter in R is:
Error: Cannot add ggproto objects together.
Did you forget to add this object to a ggplot object?
This error typically occurs when you attempt to create a visualization using the package but forget to add a plus (+) sign somewhere in the syntax.
This tutorial shares exactly how to fix this error.
How to Reproduce the Error
Suppose we have the following data frame in R that shows the number of total sales and customers that a store receives during 10 different days:
#create data frame
df <- data.frame(day = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10),
sales = c(8, 8, 7, 6, 7, 8, 9, 12, 14, 18),
customers = c(4, 6, 6, 4, 6, 7, 8, 9, 12, 13))
#view data frame
df
day sales customers
1 1 8 4
2 2 8 6
3 3 7 6
4 4 6 4
5 5 7 6
6 6 8 7
7 7 9 8
8 8 12 9
9 9 14 12
10 10 18 13
Now suppose we attempt to create a line chart to visualize the sales and customers during each of the 10 days:
library(ggplot2)
#attempt to create plot with two lines
ggplot(df, aes(x = day))
geom_line(aes(y = sales, color = 'sales')) +
geom_line(aes(y = customers, color = 'customers'))
Error: Cannot add ggproto objects together.
Did you forget to add this object to a ggplot object?
We receive an error that tell us we cannot add ggproto objects together.
How to Fix the Error
The way to fix this error is to simply add a plus sign (+) at the end of the first line, which we forgot to do the first time:
library(ggplot2)
#create plot with two lines
ggplot(df, aes(x = day)) +
geom_line(aes(y = sales, color = 'sales')) +
geom_line(aes(y = customers, color = 'customers'))
The result is a plot with two lines that shows the total customers and sales during this 10-day period.
Notice that we don’t receive an error this time because we used a plus sign (+) at the end of the first line.
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