# curve

##### Draw Function Plots

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

. `curve`

can plot also an expression in the variable
`xname`

, default `x`.

- Keywords
- hplot

##### 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 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`

.

##### Value

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`

.

##### See Also

##### Examples

`library(graphics)`

```
# NOT RUN {
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 package graphics, version 3.6.2, License: Part of R 3.6.2*

### Community examples

**chengh3@qq.com**at Nov 14, 2018 graphics v3.5.1

curve(0.0201124*exp(0.900322*(x-2008),from=2008,to=2018))