spearman's rho plot

Question

I have done a Spearman correlation and need a scatterplot with a line matching the Spearman's rho. How can I do this?

> db
   id Var_1  Var_2  Var_3 Var_4
1   5  8.17  83.08  10.28 19.81
2   6  4.06  42.30   2.72 28.92
3  17  6.06  62.43   4.55 23.10
4  18 10.88  61.12  15.96 23.50
5  27 10.67  49.29   1.99 19.50
6  30  2.85  57.22  13.05 19.90
7  37  7.26  53.79  13.91 22.30
8  40 28.77 146.45 558.33 33.10
9  41  5.26  42.87   7.03 23.40
10 42 21.38  70.32  12.60 20.00

> cor.test(db$Var_4, db$Var_2, method=c("spearman"))

    Spearman's rank correlation rho

data:  db$Var_4 and db$Var_2
S = 168, p-value = 0.9728
alternative hypothesis: true rho is not equal to 0
sample estimates:
        rho 
-0.01818182 

Answer 1

Use ‘abline’ to manually add lines to your plot.

db <- read.table(header=TRUE, text="
   id Var_1  Var_2  Var_3 Var_4
5  8.17  83.08  10.28 19.81
6  4.06  42.30   2.72 28.92
17  6.06  62.43   4.55 23.10
18 10.88  61.12  15.96 23.50
27 10.67  49.29   1.99 19.50
30  2.85  57.22  13.05 19.90
37  7.26  53.79  13.91 22.30
40 28.77 146.45 558.33 33.10
41  5.26  42.87   7.03 23.40
42 21.38  70.32  12.60 20.00")

rho <- cor(db$Var_4, db$Var_2, method=c("spearman"))

Since Spearman's correlation coefficient is based on the ranks, we transform the data to ranks before plotting.

db.rank <- dplyr::mutate(db, across(starts_with("Var"), rank))

a <- mean(db.rank$Var_2)

plot(db.rank$Var_4, db.rank$Var_2,
     main="Scatter plot of ranks\nSpearman's correlation: -0.15"))
abline(a=a, b=rho, col="red")

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