function in R Language is used to add one or more straight lines to a graph. The **abline()**`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 lineh: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.