Adding Straight Lines to a Plot in R Programming – abline() Function

abline() function in R Language is used to add one or more straight lines to a graph. The abline() function can be used to add vertical, horizontal or regression lines to plot.

Syntax:
abline(a=NULL, b=NULL, h=NULL, v=NULL, …)

Parameters:
a, b: It specifies the intercept and the slope of the line
h: specifies y-value for horizontal line(s)
v: specifies x-value(s) for vertical line(s)

Returns: a straight line in the plot

Example 1: To add a vertical line to the plot

# add line to square plot
# first example : Add one line
plot(cars)
abline(v = 16, col = "darkgreen")

# second example : add 2 lines
# addline to square plot
# change line colors, sizes and types
plot(cars)
abline(v = c(16, 22), col = c("darkgreen", "blue"),
                      lty = c(1, 2), lwd = c(1, 3))

# third example
set.seed(1200); mydata<-rnorm(180)
hist(mydata, col="darkgreen")

# lwd=line width, lty =linetype
abline(v = mean(mydata), col = "blue", lwd = 4, lty = 4)

Output:

Here, in above example straight line is added using abline() to different graphical plots

Example 2: To add a horizontal line

# R program to add a horizontal line
# to a plot

# Creating a plot
plot(cars)

# Calling abline() function
abline(h = 60, col = "darkgreen")

Output:

In above example abline() Function draws an horizontal line on the current plot at the specified ‘x’ coordinates.

Example 3: To add a regression line

par(mgp = c(2, 1, 0), mar = c(3, 3, 1, 1))

# Fit regression line
require(stats)
reg<-lm(dist ~ speed, data = cars)
coeff = coefficients(reg)

# equation of the line :
eq = paste0("y = ", round(coeff[1], 1), "*x ",
                    round(coeff[2], 1))

# plot
plot(cars, main = eq)
abline(reg, col = "darkgreen")

Output:

In the above example, straight-line is added using the line equation and abline() function and plot relation between speed and distance.