The function abline() with R

The function abline()

Syntax of abline() 

abline(a = NULL, b = NULL, h = NULL, v = NULL, reg = NULL,
       coef = NULL, col = NULL, lty = NULL, lwd = NULL, ...)

• a: Intercept of the line (for y = a + bx).
• b: Slope of the line (for y = a + bx).
• h: Numeric value(s) for horizontal lines (y = h).
• v: Numeric value(s) for vertical lines (x = v).
• reg: A regression object (e.g., from lm()).
• coef: Coefficients of a line (a and b).
• col: Color of the line.
• lty: Line type (e.g., 1 for solid, 2 for dashed).
• lwd: Line width.
• …: Additional graphical parameters.

Examples

Adding a Horizontal Line 

x <- 1:10
y <- x^2
plot(x, y, main = "Plot with Horizontal Line", xlab = "X", ylab = "Y")
abline(h = 50, col = "blue", lty = 2, lwd = 2)

• h = 50: Adds a horizontal line at y = 50.
• col = “blue”: Line color.
• lty = 2: Dashed line.
• lwd = 2: Line width.

Adding a Vertical Line 

plot(x, y, main = "Plot with Vertical Line", xlab = "X", ylab = "Y")
abline(v = 5, col = "red", lty = 3, lwd = 2)

• v = 5: Adds a vertical line at x = 5.
• col = “red”: Line color.
• lty = 3: Dotted line.
• lwd = 2: Line width.

Adding a Line with Slope and Intercept 

plot(x, y, main = "Plot with Line by Slope and Intercept", xlab = "X", ylab = "Y")
abline(a = 10, b = 1, col = "green", lty = 1, lwd = 2)

• a = 10: y-intercept.
• b = 1: Slope of the line.
• col = “green”: Line color.
• lty = 1: Solid line.
• lwd = 2: Line width.

Adding a Regression Line 

model <- lm(y ~ x)
plot(x, y, main = "Plot with Regression Line", xlab = "X", ylab = "Y")
abline(model, col = "purple", lty = 1, lwd = 2)

• model: Linear model object.
• abline(model): Adds the regression line from the model.
• col = “purple”: Line color.
• lty = 1: Solid line.
• lwd = 2: Line width.

Adding Multiple Lines 

plot(x, y, main = "Plot with Multiple Lines", xlab = "X", ylab = "Y")
abline(h = c(20, 60), col = c("blue", "red"), lty = c(2, 3), lwd = 2)

• h = c(20, 60): Adds two horizontal lines.
• col = c(“blue”, “red”): Colors for each line.
• lty = c(2, 3): Line types for each line.
• lwd = 2: Line width for both lines.

Notes

• abline() works best with simple linear functions. For more complex lines or shapes, consider using lines() or curve().
• Customization: You can combine abline() with other functions (like points() or text()) for more complex plots.