abline, segments and arrows

Base R provides several functions to add segments and arrows to the plots. In this guide we are going to use the following sample plot:

# Sample data
set.seed(132)
x <- seq(1, 10, by = 0.05)
y <- x ^ 2 + rnorm(x, sd = 10)

# Plotting function used in the examples below
splot <- function(x, y) {
  plot(x, y, pch = ifelse(x > 8, 23, ifelse(y < 0, 24, 21)),
       bg = ifelse(x > 8, "deepskyblue", ifelse(y < 0 , "orange", "lightgreen")))
}

The abline function

The abline function allows drawing lines, such as horizontal lines (h argument), vertical lines (v argument), lines based on a intersection and a slope (a and b arguments) or for plotting a regression line.

Horizontal line

The h argument allows you to set the Y-axis value where to draw a horizontal line.

splot(x, y) # Sample function

# Horizontal line at Y = 0
abline(h = 0)

Horizontal line in R with abline function

Vertical line

The v argument allows you to set the X-axis values where to draw vertical lines.

splot(x, y)

# Vertical line at X = 8
abline(v = 8)

Vertical line in R with abline

Horizontal and vertical line at the same time

You can also add vertical and horizontal lines at the same time specifying both arguments.

splot(x, y)

# Horizontal line at Y = 1 and vertical at X = 8
abline(h = 1, v = 8)

abline function in R

Intercept and slope

In addition to horizontal or vertical lines, you can also specify an intercept with the argument a and the slope of the line with the argument b.

splot(x, y)

abline(a = -15, # Intercept
       b = 10)  # Slope

Diagonal line in R with abline function setting intercept and slope

Regression line

The abline function also allows drawing linear regression lines from a model.

splot(x, y)

abline(lm(y ~ x)) # Linear regression

Adding a regression line in R with abline function

Line customization

The same customization of lines in base R can be applied to this and the other functions of this guide.

splot(x, y)

abline(h = 0, v = 8,
       col = c("red", "green"),
       lwd = 2,
       lty = 2:3)

abline function color, width and type customization

The segments function

This function is very similar to abline, but you can specify the starting and end points of the lines with x0, x1, y0 and y1 arguments.

segments function in R

splot(x, y)

segments(x0 = 1,
         x1 = 3,
         y0 = 0,
         y1 = 0,
         lwd = 2,
         col = "red") 

segments(x0 = 8,
         x1 = 8,
         y0 = 40,
         y1 = 100,
         lwd = 2,
         col = "orange") 

The arrows function

You can set the arrows the same way as segments, specifying the start and end of the arrow on each axis.

splot(x, y)

arrows(x0 = 3,
       x1 = 7,
       y0 = 40,
       y1 = 90) 

arrows function in R

In addition to modifying the color, width or line type, the arrows function also allows customizing the length and the angle of the arrowhead.

splot(x, y)

arrows(x0 = 3, x1 = 7,
       y0 = 40, y1 = 90,
       length = 0.1,
       angle = 20)  

arrows customization in R

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See also