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"))

a <- mean(db$Var_2)

plot(db$Var_4, db$Var_2)

abline(a=a, b=rho, col="red")

Edit: I was hesitant to post this after reading the comments, but I believe it answers the question. The correlation is very weak, and the red line shows this, unlike the default regression line in the ggplot shown in the question. Other lines could be added for visual comparison.