## Add Legends to Plots

## Usage

    legend(x, y, legend, fill, col = "black", lty, pch, bty = "o",
           bg = par("bg"), cex = 1, xjust = 0, yjust = 1,
           x.intersp = NULL, y.intersp = NULL, text.width = NULL,
           merge = FALSE)

## Arguments

`x` | the x location of the legend.
`y` | the y location of the legend.
`legend` | a vector of text values or an `expression` of length >= 1 to appear in the legend.
`fill` | if specied, this argument will cause boxes filled with the specified colors to appear beside the legend text.
`col` | the color of points or lines appearing in the legend.
`lty` | the line types for lines appearing in the legend. Must be specified for line drawing.
`pch` | the plotting symbols appearing in the legend, either as vector of 1-character strings, or one (multi character) string. Must be specified for symbol drawing.
`bty` | the type of box to be drawn around the legend.
`bg` | the background color for the legend box.
`cex` | character expansion factor relative to current `par("cex")`.
`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 as `x.intersp` for vertical (y) line distances.
`text.width` | the width of the legend text in x (`"user"`) coordinates. Defaults to the proper value computed by `strwidth(legend)`.
`merge` | logical; if `TRUE`, ``merge'' points and lines but not filled boxes.

## Description

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

The default values for `x.intersp` and `y.intersp` are computed from `Cx <- cex * par('cex')`.

``Attribute'' arguments such as `col`, `lty`, `pch`, etc, are recycled if necessary.

## See Also

`plot`, `barplot` which uses `legend()`, and `text` for more examples of math expressions.

## Examples

    # Run the example in `?matplot' or the following:
    matplot(c(1,8), c(0,4.5), type = "n", xlab = "Length", ylab = "Width",
            main = "Petal and Sepal Dimensions in Iris Blossoms")
    leg.txt <- c("    Setosa Petals", "    Setosa Sepals",
                 "Versicolor Petals", "Versicolor Sepals")
    y.leg <- c( 4, 3, 2.2, 1.5, .9)
    cexv  <- c( 1, .8, .6, .4, .25)
    for (i in seq(cexv))
      legend(1, y.leg[i], leg.txt, pch = "sSvV", col = c(1, 3), cex = cexv[i])

    # `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(-1,3,4), merge = TRUE)",
          cex.main = 1.1)
    legend(-1, 1.9, c("sin", "cos", "tan"), col = c(3,4,6),
           lty = c(2, -1, 1), pch = c(-1, 3, 4), merge = TRUE, bg='gray90')

    ##-- Math expressions:
    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
    legend(-3, .9, ex.cs1, lty=1:2, col=2:3)

    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))
    legend(4,30, ex12, col = 2:3, lty=2:3) # lwd = 2 does not yet work in PS

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