How to make concentric circles in r Plot

Question

I need to make concentric circles, preferably displaced from the origin, in function plot() to serve as a visual reference to a function. I tried the abline(), but I was unsuccessful.

Answer 1

Use the command draw.circle package plotrix:

x <- seq(-3, 3, by=0.01)
y <- x^2
plot(y ~ x, asp=1, type="l")

library(plotrix)
draw.circle(1, 1, 1, border="red")
draw.circle(1, 1, 2, border="green")
draw.circle(1, 1, 3, border="blue")

The syntax of the command draw.circle is quite simple:

• the first argument is the coordinate of the center of the circle on the x-axis

• the second argument is the coordinate of the center of the circle on the y-axis

• the third argument is the radius of the circle

I altered their edge with the argument border just to emphasize the difference between the drawn circles. By default, their border is black.

Answer 2

Building on Marcus Nunes' excellent response, you could also use the function symbols() package graphics.

Just remember to use add=TRUE (for him to add the circle to a preexisting Plot) and inches = FALSE, so that the natural range of radius of the circle, informed in circles, is that of the axis x.

x <- seq(-3, 3, by=0.01)
y <- x^2
plot(y ~ x, asp=1, type="l")

symbols(1, 1, circles=1, inches=FALSE, fg="red", add=TRUE)
symbols(1, 1, 2, inches=FALSE, fg="green", add=TRUE)  # circles é o 3º arg
symbols(1, 1, 3, inches=FALSE, fg="blue", add=TRUE)

Abs!