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% File src/library/graphics/man/legend.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2022 R Core Team
% Distributed under GPL 2 or later

\name{legend}
\alias{legend}
\title{Add Legends to Plots}
\usage{
legend(x, y = NULL, legend, fill = NULL, col = par("col"),
       border = "black", lty, lwd, pch,
       angle = 45, density = NULL, bty = "o", bg = par("bg"),
       box.lwd = par("lwd"), box.lty = par("lty"), box.col = par("fg"),
       pt.bg = NA, cex = 1, pt.cex = cex, pt.lwd = lwd,
       xjust = 0, yjust = 1, x.intersp = 1, y.intersp = 1,
       adj = c(0, 0.5), text.width = NULL, text.col = par("col"),
       text.font = NULL, merge = do.lines && has.pch, trace = FALSE,
       plot = TRUE, ncol = 1, horiz = FALSE, title = NULL,
       inset = 0, xpd, title.col = text.col[1], title.adj = 0.5,
       title.cex = cex[1], title.font = text.font[1],
       seg.len = 2)
}
\description{
  This function can be used to add legends to plots.  Note that a call
  to the function \code{\link{locator}(1)} can be used in place of the \code{x}
  and \code{y} arguments.
}
\arguments{
  \item{x, y}{the x and y co-ordinates to be used to position the legend.
    They can be specified by keyword or in any way which is accepted by
    \code{\link{xy.coords}}: See \sQuote{Details}.}
  \item{legend}{a character or \link{expression} vector
    of length \eqn{\ge 1} to appear in the legend.  Other
    objects will be coerced by \code{\link{as.graphicsAnnot}}.}
  \item{fill}{if specified, this argument will cause boxes filled
    with the specified colors (or shaded in the specified colors)
    to appear beside the legend text.}
  \item{col}{the color of points or lines appearing in the legend.}
  \item{border}{the border color for the boxes (used only if
    \code{fill} is specified).}
  \item{lty, lwd}{the line types and widths for lines appearing in the
    legend.  One of these two \emph{must} be specified for line drawing.}
  \item{pch}{the plotting symbols appearing in the legend, as
    numeric vector or a vector of 1-character strings (see
    \code{\link{points}}). Unlike \code{points}, this can all be
    specified as a single multi-character string.  \emph{Must} be
    specified for symbol drawing.}
  \item{angle}{angle of shading lines.}
  \item{density}{the density of shading lines, if numeric and
    positive. If \code{NULL} or negative or \code{NA} color filling
    is assumed.}
  \item{bty}{the type of box to be drawn around the legend.  The allowed
    values are \code{"o"} (the default) and \code{"n"}.}
  \item{bg}{the background color for the legend box.  (Note that this is
    only used if \code{bty != "n"}.)}
  \item{box.lty, box.lwd, box.col}{the line type, width and color for
    the legend box (if \code{bty = "o"}).}
  \item{pt.bg}{the background color for the \code{\link{points}},
    corresponding to its argument \code{bg}.}
  \item{cex}{character expansion factor \bold{relative} to current
    \code{par("cex")}.  Used for text, and provides the default for
    \code{pt.cex}.}
  \item{pt.cex}{expansion factor(s) for the points.}
  \item{pt.lwd}{line width for the points, defaults to the one for
    lines, or if that is not set, to \code{par("lwd")}.}
  \item{xjust}{how the legend is to be justified relative to the legend
    x location.  A value of 0 means left justified, 0.5 means centered
    and 1 means right justified.}
  \item{yjust}{the same as \code{xjust} for the legend y location.}
  \item{x.intersp}{character interspacing factor for horizontal (x) spacing
    between symbol and legend text.}
  \item{y.intersp}{vertical (y) distances (in lines of text shared above/below each legend entry).
    A vector with one element for each row of the legend can be used.}
  \item{adj}{numeric of length 1 or 2; the string adjustment for legend
    text.  Useful for y-adjustment when \code{labels} are
    \link{plotmath} expressions.}
  \item{text.width}{the width of the legend text in x (\code{"user"})
    coordinates.  (Should be positive even for a reversed x axis.)
    Can be a single positive numeric value (same width for each column of the legend),
    a vector (one element for each column of the legend),
    \code{NULL} (default) for computing a proper maximum value of \code{\link{strwidth}(legend)}),
    or \code{NA} for computing a proper column wise maximum value of \code{\link{strwidth}(legend)}).}
  \item{text.col}{the color used for the legend text.}
  \item{text.font}{the font used for the legend text, see \code{\link{text}}.}
  \item{merge}{logical; if \code{TRUE}, merge points and lines but
    not filled boxes.  Defaults to \code{TRUE} if there are points and lines.}
  \item{trace}{logical; if \code{TRUE}, shows how \code{legend} does all
    its magical computations.}
  \item{plot}{logical.  If \code{FALSE}, nothing is plotted but the
    sizes are returned.}
  \item{ncol}{the number of columns in which to set the legend items
    (default is 1, a vertical legend).}
  \item{horiz}{logical; if \code{TRUE}, set the legend horizontally
    rather than vertically (specifying \code{horiz} overrides the \code{ncol}
    specification).}
  \item{title}{a character string or length-one expression giving a
    title to be placed at the top of the legend.  Other objects will be
    coerced by \code{\link{as.graphicsAnnot}}.}
  \item{inset}{inset distance(s) from the margins as a fraction of the
    plot region when legend is placed by keyword.}
  \item{xpd}{if supplied, a value of the \link{graphical parameter} \code{xpd}
    to be used while the legend is being drawn.}
  \item{title.col}{color for \code{title}, defaults to \code{text.col[1]}.}
  \item{title.adj}{horizontal adjustment for \code{title}: see the help
    for \code{\link{par}("adj")}.}
  \item{title.cex}{expansion factor(s) for the title, defaults to \code{cex[1]}.}
  \item{title.font}{the font used for the legend title, defaults to \code{text.font[1]},
    see \code{\link{text}}.}
  \item{seg.len}{the length of lines drawn to illustrate \code{lty}
    and/or \code{lwd} (in units of character widths).}
}
\details{
  Arguments \code{x}, \code{y}, \code{legend} are interpreted in a
  non-standard way to allow the coordinates to be specified \emph{via}
  one or two arguments.  If \code{legend} is missing and \code{y} is not
  numeric, it is assumed that the second argument is intended to be
  \code{legend} and that the first argument specifies the coordinates.

  The coordinates can be specified in any way which is accepted by
  \code{\link{xy.coords}}.  If this gives the coordinates of one point,
  it is used as the top-left coordinate of the rectangle containing the
  legend.  If it gives the coordinates of two points, these specify
  opposite corners of the rectangle (either pair of corners, in any
  order).

  The location may also be specified by setting \code{x} to a single
  keyword from the list \code{"bottomright"}, \code{"bottom"},
  \code{"bottomleft"}, \code{"left"}, \code{"topleft"},
  \code{"top"}, \code{"topright"}, \code{"right"} and
  \code{"center"}. This places the legend on the inside of the plot
  frame at the given location. Partial argument matching is used.  The
  optional \code{inset} argument specifies how far the legend is inset
  from the plot margins.  If a single value is given, it is used for
  both margins; if two values are given, the first is used for \code{x}-
  distance, the second for \code{y}-distance.

  Attribute arguments such as \code{col}, \code{pch}, \code{lty}, etc,
  are recycled if necessary: \code{merge} is not.  Set entries of
  \code{lty} to \code{0} or set entries of \code{lwd} to \code{NA} to
  suppress lines in corresponding legend entries; set \code{pch}
  values to \code{NA} to suppress points.

  Points are drawn \emph{after} lines in order that they can cover the
  line with their background color \code{pt.bg}, if applicable.

  See the examples for how to right-justify labels.

  Since they are not used for Unicode code points, values \code{-31:-1}
  are silently omitted, as are \code{NA} and \code{""} values.
}
\value{
  A list with list components
  \item{rect}{a list with components
    \describe{
      \item{\code{w}, \code{h}}{positive numbers giving \bold{w}idth and
        \bold{h}eight of the legend's box.}
      \item{\code{left}, \code{top}}{x and y coordinates of upper left
        corner of the box.}
    }
  }
  \item{text}{a list with components
    \describe{
      \item{\code{x}, \code{y}}{numeric vectors of length \code{length(legend)},
        giving the x and y coordinates of the legend's text(s).}
    }
  }
  returned invisibly.
}
\references{
  \bibshow{R:Becker+Chambers+Wilks:1988}

  \bibshow{R:Murrell:2005}
}
\seealso{
  \code{\link{plot}}, \code{\link{barplot}} which uses \code{legend()}, and
  \code{\link{text}} for more examples of math expressions.
}
\examples{
## Run the example in '?matplot' or the following:
leg.txt <- c("Setosa     Petals", "Setosa     Sepals",
             "Versicolor Petals", "Versicolor Sepals")
y.leg <- c(4.5, 3, 2.1, 1.4, .7)
cexv  <- c(1.2, 1, 4/5, 2/3, 1/2)
matplot(c(1, 8), c(0, 4.5), type = "n", xlab = "Length", ylab = "Width",
        main = "Petal and Sepal Dimensions in Iris Blossoms")
for (i in seq(cexv)) {
  text  (1, y.leg[i] - 0.1, paste("cex=", formatC(cexv[i])), cex = 0.8, adj = 0)
  legend(3, y.leg[i], leg.txt, pch = "sSvV", col = c(1, 3), cex = cexv[i])
}
## cex *vector* [in R <= 3.5.1 has 'if(xc < 0)' w/ length(xc) == 2]
legend("right", leg.txt, pch = "sSvV", col = c(1, 3),
       cex = 1+(-1:2)/8, trace = TRUE)# trace: show computed lengths & coords

## 'merge = TRUE' for merging lines & points:
x <- seq(-pi, pi, length.out = 65)
for(reverse in c(FALSE, TRUE)) {  ## normal *and* reverse axes:
  F <- if(reverse) rev else identity
  plot(x, sin(x), type = "l", col = 3, lty = 2,
       xlim = F(range(x)), ylim = F(c(-1.2, 1.8)))
  points(x, cos(x), pch = 3, col = 4)
  lines(x, tan(x), type = "b", lty = 1, pch = 4, col = 6)
  title("legend('top', lty = c(2, -1, 1), pch = c(NA, 3, 4), merge = TRUE)",
        cex.main = 1.1)
  legend("top", c("sin", "cos", "tan"), col = c(3, 4, 6),
       text.col = "green4", lty = c(2, -1, 1), pch = c(NA, 3, 4),
       merge = TRUE, bg = "gray90", trace=TRUE)
  \dontshow{ if(!reverse && interactive()) {
    cat("waiting .."); Sys.sleep(2); cat(" done\n") }
  }
} # for(..)

## right-justifying a set of labels: thanks to Uwe Ligges
x <- 1:5; y1 <- 1/x; y2 <- 2/x
plot(rep(x, 2), c(y1, y2), type = "n", xlab = "x", ylab = "y")
lines(x, y1); lines(x, y2, lty = 2)
temp <- legend("topright", legend = c(" ", " "),
               text.width = strwidth("1,000,000"),
               lty = 1:2, xjust = 1, yjust = 1, inset = 1/10,
               title = "Line Types", title.cex = 0.5, trace=TRUE)
text(temp$rect$left + temp$rect$w, temp$text$y,
     c("1,000", "1,000,000"), pos = 2)


##--- log scaled Examples ------------------------------
leg.txt <- c("a one", "a two")

par(mfrow = c(2, 2))
for(ll in c("","x","y","xy")) {
  plot(2:10, log = ll, main = paste0("log = '", ll, "'"))
  abline(1, 1)
  lines(2:3, 3:4, col = 2)
  points(2, 2, col = 3)
  rect(2, 3, 3, 2, col = 4)
  text(c(3,3), 2:3, c("rect(2,3,3,2, col=4)",
                      "text(c(3,3),2:3,\"c(rect(...)\")"), adj = c(0, 0.3))
  legend(list(x = 2,y = 8), legend = leg.txt, col = 2:3, pch = 1:2,
         lty = 1)  #, trace = TRUE)
} #      ^^^^^^^ to force lines -> automatic merge=TRUE
par(mfrow = c(1,1))

##-- Math expressions:  ------------------------------
x <- seq(-pi, pi, length.out = 65)
plot(x, sin(x), type = "l", col = 2, xlab = expression(phi),
     ylab = expression(f(phi)))
abline(h = -1:1, v = pi/2*(-6:6), col = "gray90")
lines(x, cos(x), col = 3, lty = 2)
ex.cs1 <- expression(plain(sin) * phi,  paste("cos", phi))  # 2 ways
utils::str(legend(-3, .9, ex.cs1, lty = 1:2, plot = FALSE,
           adj = c(0, 0.6)))  # adj y !
legend(-3, 0.9, ex.cs1, lty = 1:2, col = 2:3,  adj = c(0, 0.6))

require(stats)
x <- rexp(100, rate = .5)
hist(x, main = "Mean and Median of a Skewed Distribution")
abline(v = mean(x),   col = 2, lty = 2, lwd = 2)
abline(v = median(x), col = 3, lty = 3, lwd = 2)
ex12 <- expression(bar(x) == sum(over(x[i], n), i == 1, n),
                   hat(x) == median(x[i], i == 1, n))
utils::str(legend(4.1, 30, ex12, col = 2:3, lty = 2:3, lwd = 2))

## 'Filled' boxes -- see also example(barplot) which may call legend(*, fill=)
barplot(VADeaths)
legend("topright", rownames(VADeaths), fill = gray.colors(nrow(VADeaths)))

## Using 'ncol'
x <- 0:64/64
for(R in c(identity, rev)) { # normal *and* reverse x-axis works fine:
  xl <- R(range(x)); x1 <- xl[1]
matplot(x, outer(x, 1:7, function(x, k) sin(k * pi * x)), xlim=xl,
        type = "o", col = 1:7, ylim = c(-1, 1.5), pch = "*")
op <- par(bg = "antiquewhite1")
legend(x1, 1.5, paste("sin(", 1:7, "pi * x)"), col = 1:7, lty = 1:7,
       pch = "*", ncol = 4, cex = 0.8)
legend("bottomright", paste("sin(", 1:7, "pi * x)"), col = 1:7, lty = 1:7,
       pch = "*", cex = 0.8)
legend(x1, -.1, paste("sin(", 1:4, "pi * x)"), col = 1:4, lty = 1:4,
       ncol = 2, cex = 0.8)
legend(x1, -.4, paste("sin(", 5:7, "pi * x)"), col = 4:6,  pch = 24,
       ncol = 2, cex = 1.5, lwd = 2, pt.bg = "pink", pt.cex = 1:3)
par(op)
  \dontshow{ if(interactive() && identical(R, identity)) {
     cat("waiting .."); Sys.sleep(2); cat(" done\n") }}
} # for(..)

## point covering line :
y <- sin(3*pi*x)
plot(x, y, type = "l", col = "blue",
    main = "points with bg & legend(*, pt.bg)")
points(x, y, pch = 21, bg = "white")
legend(.4,1, "sin(c x)", pch = 21, pt.bg = "white", lty = 1, col = "blue")

## legends with titles at different locations
plot(x, y, type = "n")
legend("bottomright", "(x,y)", pch=1, title= "bottomright")
legend("bottom",      "(x,y)", pch=1, title= "bottom")
legend("bottomleft",  "(x,y)", pch=1, title= "bottomleft")
legend("left",        "(x,y)", pch=1, title= "left")
legend("topleft",     "(x,y)", pch=1, title= "topleft, inset = .05", inset = .05)
legend("top",         "(x,y)", pch=1, title= "top")
legend("topright",    "(x,y)", pch=1, title= "topright, inset = .02",inset = .02)
legend("right",       "(x,y)", pch=1, title= "right")
legend("center",      "(x,y)", pch=1, title= "center")

# using text.font (and text.col):
op <- par(mfrow = c(2, 2), mar = rep(2.1, 4))
c6 <- terrain.colors(10)[1:6]
for(i in 1:4) {
   plot(1, type = "n", axes = FALSE, ann = FALSE); title(paste("text.font =",i))
   legend("top", legend = LETTERS[1:6], col = c6,
          ncol = 2, cex = 2, lwd = 3, text.font = i, text.col = c6)
}
par(op)

# using text.width for several columns
plot(1, type="n")
legend("topleft", c("This legend", "has", "equally sized", "columns."),
       pch = 1:4, ncol = 4)
legend("bottomleft", c("This legend", "has", "optimally sized", "columns."),
       pch = 1:4, ncol = 4, text.width = NA)
legend("right", letters[1:4], pch = 1:4, ncol = 4,
       text.width = 1:4 / 50)
}
\keyword{aplot}