# curve {graphics}

### Description

Draws a curve corresponding to a function over the interval `[from, to]`

. `curve`

can plot also an expression in the variable `xname`

, default x.

### Usage

curve(expr, from = NULL, to = NULL, n = 101, add = FALSE, type = "l", xname = "x", xlab = xname, ylab = NULL, log = NULL, xlim = NULL, ...) ## S3 method for class 'function': plot((x, y = 0, to = 1, from = y, xlim = NULL, ylab = NULL, ...))

### Arguments

- expr
- The name of a function, or a call or an expression written as a function of
`x`

which will evaluate to an object of the same length as`x`

. - x
- a ‘vectorizing’ numeric R function.
- y
- alias for
`from`

for compatibility with`plot`

- from, to
- the range over which the function will be plotted.
- n
- integer; the number of x values at which to evaluate.
- add
- logical; if
`TRUE`

add to an already existing plot; if`NA`

start a new plot taking the defaults for the limits and log-scaling of the x-axis from the previous plot. Taken as`FALSE`

(with a warning if a different value is supplied) if no graphics device is open. - xlim
`NULL`

or a numeric vector of length 2; if non-`NULL`

it provides the defaults for`c(from, to)`

and, unless`add = TRUE`

, selects the x-limits of the plot -- see`plot.window`

.- type
- plot type: see
`plot.default`

. - xname
- character string giving the name to be used for the x axis.
- xlab, ylab, log, ...
- labels and graphical parameters can also be specified as arguments. See ‘Details’ for the interpretation of the default for
`log`

.For the

`"function"`

method of`plot`

,`...`

can include any of the other arguments of`curve`

, except`expr`

.

### Details

The function or expression `expr`

(for `curve`

) or function `x`

(for `plot`

) is evaluated at `n`

points equally spaced over the range `[from, to]`

. The points determined in this way are then plotted.

If either `from`

or `to`

is `NULL`

, it defaults to the corresponding element of `xlim`

if that is not `NULL`

.

What happens when neither `from`

/`to`

nor `xlim`

specifies both x-limits is a complex story. For `plot(<function>)`

and for `curve(add = FALSE)`

the defaults are (0, 1). For `curve(add = NA)`

and `curve(add = TRUE)`

the defaults are taken from the x-limits used for the previous plot. (This differs from versions of R prior to 2.14.0.)

The value of `log`

is used both to specify the plot axes (unless `add = TRUE`

) and how ‘equally spaced’ is interpreted: if the x component indicates log-scaling, the points at which the expression or function is plotted are equally spaced on log scale.

The default value of `log`

is taken from the current plot when `add = TRUE`

, whereas if `add = NA`

the x component is taken from the existing plot (if any) and the y component defaults to linear. For `add = FALSE`

the default is `""`

This used to be a quick hack which now seems to serve a useful purpose, but can give bad results for functions which are not smooth.

For expensive-to-compute `expr`

essions, you should use smarter tools.

The way `curve`

handles `expr`

has caused confusion. It first looks to see if `expr`

is a name (also known as a symbol), in which case it is taken to be the name of a function, and `expr`

is replaced by a call to `expr`

with a single argument with name given by `xname`

. Otherwise it checks that `expr`

is either a call or an expression, and that it contains a reference to the variable given by `xname`

(using `all.vars`

): anything else is an error. Then `expr`

is evaluated in an environment which supplies a vector of name given by `xname`

of length `n`

, and should evaluate to an object of length `n`

. Note that this means that `curve(x, ...)`

is taken as a request to plot a function named `x`

(and it is used as such in the `function`

method for `plot`

).

The `plot`

method can be called directly as `plot.function`

.

### Values

A list with components `x`

and `y`

of the points that were drawn is returned invisibly.

### Warning

For historical reasons, `add`

is allowed as an argument to the `"function"`

method of `plot`

, but its behaviour may surprise you. It is recommended to use `add`

only with `curve`

.

### Examples

plot(qnorm) # default range c(0, 1) is appropriate here, # but end values are -/+Inf and so are omitted. plot(qlogis, main = "The Inverse Logit : qlogis()") abline(h = 0, v = 0:2/2, lty = 3, col = "gray") curve(sin, -2*pi, 2*pi, xname = "t") curve(tan, xname = "t", add = NA, main = "curve(tan) --> same x-scale as previous plot") op <- par(mfrow = c(2, 2)) curve(x^3 - 3*x, -2, 2) curve(x^2 - 2, add = TRUE, col = "violet") ## simple and advanced versions, quite similar: plot(cos, -pi, 3*pi) curve(cos, xlim = c(-pi, 3*pi), n = 1001, col = "blue", add = TRUE) chippy <- function(x) sin(cos(x)*exp(-x/2)) curve(chippy, -8, 7, n = 2001) plot (chippy, -8, -5) for(ll in c("", "x", "y", "xy")) curve(log(1+x), 1, 100, log = ll, sub = paste0("log = '", ll, "'")) par(op)

Documentation reproduced from R 3.0.2. License: GPL-2.