If you are using the ggplot2 package to visualize data and forget to use a plus sign in the code, you will encounter the error: cannot add ggproto plots together.

This tutorial will go through how to solve the error with code examples.


Table of contents

Example

Let’s look at an example of a data frame that contains the number of ice creams sold over ten days together with the temperature on each day in celsius.

df <- data.frame(day=c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10),
temperature=c(10, 18, 22, 19, 28, 15, 20, 17, 13, 30),
ice_cream_sold=c(5, 10, 40, 38, 100, 40, 120, 10, 30, 150))

df
 day temperature ice_cream_sold
1    1          10              5
2    2          18             10
3    3          22             40
4    4          19             38
5    5          28            100
6    6          15             40
7    7          20            120
8    8          17             10
9    9          13             30
10  10          30            150

Let’s try to plot temperature and ice_cream_sold as two line plots on the same figure using ggplot2.

library(ggplot2)

ggplot(df, aes(x = day))
  geom_line(aes(y = temperature, color = 'temperature')) + 
  geom_line(aes(y = ice_cream_sold, color = 'ice_cream_sold'))

Let’s run the code to see what happens:

Error in `+.gg`:
! Cannot add ggproto objects together. Did you forget to add this object to a ggplot object?

The error occurs because we are missing a + sign at the end of the first line.

Solution

We can solve this error by ensuring the plus signs are in place to create a ggplot chain. We need a + between the first ggplot() call and geom_point. Let’s look at the revised code:

library(ggplot2)

coeff <- 1

ggplot(df, aes(x = day)) + 
  geom_line(aes(y = ice_cream_sold, color = 'ice_cream_sold')) + 
  geom_line(aes(y = temperature, color = 'temperature')) 

Let’s run the code to get the result:

Line plot showing ice cream sold and temperature on each day
Line plot showing ice cream sold and temperature on each day

We successfully plotted the two variables ice_cream_sold and temperature on the same figure.

Summary

Congratulations on reading to the end of this tutorial! 

For further reading on R related errors, go to the articles: 

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Have fun and happy researching!