将图例添加到由两列组成的 ggplot

Question

I am plotting residuals from two different methods with the following code:

ggplot(df_index, aes(cases100ppl, lm_errors)) +
  geom_point(alpha=1) +
  geom_point(data=df_index, aes(cases100ppl, error), col="red", alpha=0.2) 

How can I add a legend to this?

the data has a structure like this:

code       cases100ppl   error         lm_errors
E02000001  0.05575558    0.2228769     0.1554760                
E02000002  0.11299289    0.3680860     0.4357544            
E02000003  0.11938429    0.4785204     0.3163543            
E02000004  0.10767160    0.1978992     0.3909933            
E02000005  0.11138804    0.3544542     0.3370886            
E02000007  0.09484474    0.3447380     0.3881657

Output looks something like this: Thanks!

Answer 1

You will need to change a little your data so you can use aes() to set the color and the alpha. This is a very usefull trick with ggplot (you can find ways to do it, including the one presented here, on SO posts like this one ). You can find more general informations about pivoting here in the book R for data science, chapter 12 Tidy data .

Accordingly, I pivot your dataframe to make a new variable called error_type . This new variable is then used inside aes() so the legend is created accordingly. Note that, using, dplyr pipe symbol %>% I pivot your dataframe just before entering ggplot world, without changing the original df_index object.

Then you can use scale_alpha_manual() and scale_colour_manual() to custom the color and the alpha the way you want it to be.

Here is a start:

library(dplyr)
library(tidyr)
library(ggplot2)

df_index %>% 
  pivot_longer(cols = c("error", "lm_errors"), names_to = "error_type", values_to = "error_value") %>% 
  ggplot(data = ., aes(x = cases100ppl, 
                       y = error_value, 
                       color = error_type, 
                       alpha = error_type)) + # do not forget to put alpha inside aes()!
  scale_alpha_manual(values = c("error" = 0.3, "lm_errors" = 1)) +
  geom_point()

Data:

df_index <- structure(list(code = c("E02000001", "E02000002", "E02000003", 
                                    "E02000004", "E02000005", "E02000007"), cases100ppl = c(0.05575558, 
                                                                                            0.11299289, 0.11938429, 0.1076716, 0.11138804, 0.09484474), error = c(0.2228769, 
                                                                                                                                                                  0.368086, 0.4785204, 0.1978992, 0.3544542, 0.344738), lm_errors = c(0.155476, 
                                                                                                                                                                                                                                      0.4357544, 0.3163543, 0.3909933, 0.3370886, 0.3881657)), class = "data.frame", row.names = c(NA, 
                                                                                                                                                                                                                                                                                                                                   -6L))