legend: Add Legends to Plots

Description

This function can be used to add legends to plots. Note that a call to the function locator(1) can be used in place of the x and y arguments.

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, title.adj = 0.5,
       seg.len = 2)

Arguments

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 xy.coords: See ‘Details’.

legend

a character or expression vector of length $\geq 1$ to appear in the legend. Other objects will be coerced by as.graphicsAnnot.

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.

col

the color of points or lines appearing in the legend.

border

the border color for the boxes (used only if fill is specified).

lty, lwd

the line types and widths for lines appearing in the legend. One of these two must be specified for line drawing.

pch

the plotting symbols appearing in the legend, as numeric vector or a vector of 1-character strings (see points). Unlike points, this can all be specified as a single multi-character string. Must be specified for symbol drawing.

angle

angle of shading lines.

density

the density of shading lines, if numeric and positive. If NULL or negative or NA color filling is assumed.

bty

the type of box to be drawn around the legend. The allowed values are "o" (the default) and "n".

the background color for the legend box. (Note that this is only used if bty != "n".)

box.lty, box.lwd, box.col

the line type, width and color for the legend box (if bty = "o").

pt.bg

the background color for the points, corresponding to its argument bg.

cex

character expansion factor relative to current par("cex"). Used for text, and provides the default for pt.cex.

pt.cex

expansion factor(s) for the points.

pt.lwd

line width for the points, defaults to the one for lines, or if that is not set, to par("lwd").

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.

yjust

the same as xjust for the legend y location.

x.intersp

character interspacing factor for horizontal (x) spacing.

y.intersp

the same for vertical (y) line distances.

adj

numeric of length 1 or 2; the string adjustment for legend text. Useful for y-adjustment when labels are plotmath expressions.

text.width

the width of the legend text in x ("user") coordinates. (Should be a single positive number even for a reversed x axis.) Defaults to the proper value computed by strwidth(legend).

text.col

the color used for the legend text.

text.font

the font used for the legend text, see text.

merge

logical; if TRUE, merge points and lines but not filled boxes. Defaults to TRUE if there are points and lines.

trace

logical; if TRUE, shows how legend does all its magical computations.

plot

logical. If FALSE, nothing is plotted but the sizes are returned.

ncol

the number of columns in which to set the legend items (default is 1, a vertical legend).

horiz

logical; if TRUE, set the legend horizontally rather than vertically (specifying horiz overrides the ncol specification).

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 as.graphicsAnnot.

inset

inset distance(s) from the margins as a fraction of the plot region when legend is placed by keyword.

xpd

if supplied, a value of the graphical parameter xpd to be used while the legend is being drawn.

title.col

color for title.

title.adj

horizontal adjustment for title: see the help for par("adj").

seg.len

the length of lines drawn to illustrate lty and/or lwd (in units of character widths).

Value

A list with list components

rect

a list with components

w, h: positive numbers giving width and height of the legend's box.
left, top: x and y coordinates of upper left corner of the box.

text

a list with components

x, y: numeric vectors of length length(legend), giving the x and y coordinates of the legend's text(s).

returned invisibly.

Details

Arguments x, y, legend are interpreted in a non-standard way to allow the coordinates to be specified via one or two arguments. If legend is missing and y is not numeric, it is assumed that the second argument is intended to be legend and that the first argument specifies the coordinates.

The coordinates can be specified in any way which is accepted by 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 x to a single keyword from the list "bottomright", "bottom", "bottomleft", "left", "topleft", "top", "topright", "right" and "center". This places the legend on the inside of the plot frame at the given location. Partial argument matching is used. The optional 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 x- distance, the second for y-distance.

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

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

See the examples for how to right-justify labels.

Since they are not used for Unicode code points, values -31:-1 are silently omitted, as are NA and "" values.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Murrell, P. (2005) R Graphics. Chapman & Hall/CRC Press.

Examples

Run this code

# NOT RUN {
## 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(6,1, leg.txt, pch = "sSvV", col = c(1, 3), cex = 1+(-1:2)/8)

## 'merge = TRUE' for merging lines & points:
x <- seq(-pi, pi, len = 65)
plot(x, sin(x), type = "l", ylim = c(-1.2, 1.8), col = 3, lty = 2)
points(x, cos(x), pch = 3, col = 4)
lines(x, tan(x), type = "b", lty = 1, pch = 4, col = 6)
title("legend(..., lty = c(2, -1, 1), pch = c(NA, 3, 4), merge = TRUE)",
      cex.main = 1.1)
legend(-1, 1.9, 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")

## 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,
               title = "Line Types")
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, merge = TRUE)   #, trace = TRUE)
}
par(mfrow = c(1,1))

##-- Math expressions:  ------------------------------
x <- seq(-pi, pi, len = 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 -- for more, see example(plot.factor)
op <- par(bg = "white") # to get an opaque box for the legend
plot(cut(weight, 3) ~ group, data = PlantGrowth, col = NULL,
     density = 16*(1:3))
par(op)

## Using 'ncol' :
x <- 0:64/64
matplot(x, outer(x, 1:7, function(x, k) sin(k * pi * x)),
        type = "o", col = 1:7, ylim = c(-1, 1.5), pch = "*")
op <- par(bg = "antiquewhite1")
legend(0, 1.5, paste("sin(", 1:7, "pi * x)"), col = 1:7, lty = 1:7,
       pch = "*", ncol = 4, cex = 0.8)
legend(.8,1.2, paste("sin(", 1:7, "pi * x)"), col = 1:7, lty = 1:7,
       pch = "*", cex = 0.8)
legend(0, -.1, paste("sin(", 1:4, "pi * x)"), col = 1:4, lty = 1:4,
       ncol = 2, cex = 0.8)
legend(0, -.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)

## 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)
# }

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