## Add a Straight Line to a Plot

## Usage

    abline(a, b, ...)
    abline(h=, ...)
    abline(v=, ...)
    abline(coef=, ...)
    abline(reg=, ...)

## Description

This function adds one or more straight lines through the current plot. The first form above specifies the line in intercept/slope form (alternatively `a` can be specified on its own and is taken to contain the slope and intercept in vector form). The `h=` and `v=` forms draw horizontal and vertical lines at the specified coordinates. The `coef` form specifies the line by a vector containing the slope and intercept. `reg` is a regression object which contains `reg$coef`. If it is of length 1 then the value is taken to be the slope of a line through the origin, otherwise, the first 2 values are taken to be the slope and intercept.

The graphical parameters `col` and `lty` can be specified as arguments to `abline`; see `par` for details.

## See Also

`lines` and `segments` for connected and arbitrary lines given by their endpoints. `par`.

## Examples

    data(cars)
    z <- lm(dist ~ speed, data = cars)
    plot(cars)
    abline(z)

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