Why does abline shows on the opposite side on kernel density plots?

I’m curious why the vertical line is showing at -10 and not 10 in the following example:

x <- rnorm(1e4, 0, 10)
plot(density(x))
abline(10, 1, col = "red")

>Solution :

What you need is using h=10 instead of a=10, b=1. From the helpfile you can read:

h the x-value(s) for vertical line(s).

So, to produce your desired output use the following:

abline(v=10, col = "red")