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.
1 2 3 4 5 6 7 8 9 10 11  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)

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

legend 
a character or expression vector
of length ≥ 1 to appear in the legend. Other
objects will be coerced by 
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

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

angle 
angle of shading lines. 
density 
the density of shading lines, if numeric and
positive. If 
bty 
the type of box to be drawn around the legend. The allowed
values are 
bg 
the background color for the legend box. (Note that this is
only used if 
box.lty, box.lwd, box.col 
the line type, width and color for
the legend box (if 
pt.bg 
the background color for the 
cex 
character expansion factor relative to current

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 
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 
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 yadjustment when 
text.width 
the width of the legend text in x ( 
text.col 
the color used for the legend text. 
text.font 
the font used for the legend text, see 
merge 
logical; if 
trace 
logical; if 
plot 
logical. If 
ncol 
the number of columns in which to set the legend items (default is 1, a vertical legend). 
horiz 
logical; if 
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 
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 
title.col 
color for 
title.adj 
horizontal adjustment for 
seg.len 
the length of lines drawn to illustrate 
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.
A list with list components
rect 
a list with components

text 
a list with components

returned invisibly.
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.
plot
, barplot
which uses legend()
, and
text
for more examples of math expressions.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123  ## 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")
## rightjustifying 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)

Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.