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[1], title.adj = 0.5,
title.cex = cex[1], title.font = text.font[1],
seg.len = 2)
Arguments
x , y

the x and y coordinates 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 $\ge 1$ 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 1character strings (see
points ). Unlike points , this can all be
specified as a single multicharacter 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" .

bg 
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
between symbol and legend text.

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.

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

text.width 
the width of the legend text in x ("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),
NULL (default) for computing a proper maximum value of strwidth(legend) ),
or NA for computing a proper column wise maximum value of 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 lengthone 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 , defaults to text.col[1] .

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

title.cex 
expansion factor(s) for the title, defaults to cex[1] .

title.font 
the font used for the legend title, defaults to text.font[1] ,
see text .

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

Details
Arguments x
, y
, legend
are interpreted in a
nonstandard 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 topleft 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 rightjustify labels.
Since they are not used for Unicode code points, values 31:1
are silently omitted, as are NA
and ""
values.
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.
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.
See Also
plot
, barplot
which uses legend()
, and
text
for more examples of math expressions.
Examples
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])
}
legend("right", leg.txt, pch = "sSvV", col = c(1, 3),
cex = 1+(1:2)/8, trace = TRUE)
x < seq(pi, pi, length.out = 65)
for(reverse in c(FALSE, TRUE)) {
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)
}
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)
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)
}
par(mfrow = c(1,1))
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))
utils::str(legend(3, .9, ex.cs1, lty = 1:2, plot = FALSE,
adj = c(0, 0.6)))
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))
barplot(VADeaths)
legend("topright", rownames(VADeaths), fill = gray.colors(nrow(VADeaths)))
x < 0:64/64
for(R in c(identity, rev)) {
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)
}
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")
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")
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)
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)