# Abline in R – A Quick Tutorial

Ever needed to add straight lines to an R plot? You can use abline in R to add straight lines to a plot. This R function is great for adding cutoffs or similar limits to an existing R plot.

``````# abline in r - basic syntax

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

The abline function can accept several arguments, with defaults of NULL:

• a – The intercept of the line, as a single value
• b – The slope of a line, as a single value
• h – the y-value(s) for horizontal line(s)
• v – the x-value(s) for vertical line(s).
• coef – a vector of length two giving the intercept and slope
• reg – a regression object generated by the coef method; intended to simplifying passing trend-lines from a regression calculation
• col – optional color parameter for the line
• lty – optional line type parameter
• lwd – optional line width parameter

#### Initial Setup – Examples

We’re going to use the airmiles dataset for this tutorial. This is one of the default datasets available in R.

``````> airmiles

# default plot for abline in r tutorial
> plot (airmiles)``````

As you can see, airmiles is a nice basic timeseries. It shows the growth of airline travel within the US.

#### Abline in r – Add Horizontal Lines

First, lets start by drawing a horizontal line on the graph. Maybe we are very interested in seeing when the total miles traveled is above 20,000.

``````# abline in R example - horizontal line
abline(20000,0)``````

This draws a lovely horizontal line at the y = 20000 level.

#### Abline in r – Add Vertical Lines

We can also easily add a vertical line to the graph, similar to a dropline in Excel. Perhaps our client would like to see a line at the year 1955.

``````# abline in R example - horizontal line
abline(v = 1955)``````

This draws a lovely horizontal line at the x = 1955 level.

#### Abline in r – Color and Line Types

Moving past basic black and white, we can easily adapt the abline function to draw lines in color and in different formats.

``````plot(airmiles)
abline(v=1955, col='blue', lty=2, lwd=5)``````

#### Abline in r – Multiple Lines

In this next example, we’re going to draw a range of two lines on the chart. There’s syntax that can simplify this for horizontal lines.

``````plot(airmiles)
abline(c(5000, 25000), col='blue', lty=2, lwd=4)
abline(5000, 0, col='blue', lty=2, lwd=4)
abline(25000, 0, col='blue', lty=2, lwd=4)``````

#### Abline in r – Drawing a slope

For our next example, we will demonstrate how to draw a sloped line.

A slope of 1 is a 45 degree line.

``````with(airquality, plot(Ozone, Temp))
abline(60, .5, col='blue', lty=2, lwd=4)
abline(60, 0, col='blue', lty=2, lwd=4)
abline(60, 1, col='blue', lty=2, lwd=4)``````