bxp: Draw Box Plots from Summaries

Description

bxp draws box plots based on the given summaries in z. It is usually called from within boxplot, but can be invoked directly.

Usage

bxp(z, notch = FALSE, width = NULL, varwidth = FALSE, outline = TRUE, notch.frac = 0.5, log = "", border = par("fg"), pars = NULL, frame.plot = axes, horizontal = FALSE, add = FALSE, at = NULL, show.names = NULL, ...)

Arguments

a list containing data summaries to be used in constructing the plots. These are usually the result of a call to boxplot, but can be generated in any fashion.

notch

if notch is TRUE, a notch is drawn in each side of the boxes. If the notches of two plots do not overlap then the medians are significantly different at the 5 percent level.

width

a vector giving the relative widths of the boxes making up the plot.

varwidth

if varwidth is TRUE, the boxes are drawn with widths proportional to the square-roots of the number of observations in the groups.

outline

if outline is not true, the outliers are not drawn.

notch.frac

numeric in (0,1). When notch = TRUE, the fraction of the box width that the notches should use.

border

character or numeric (vector), the color of the box borders. Is recycled for multiple boxes. Is used as default for the boxcol, medcol, whiskcol, staplecol, and outcol options (see below).

log

character, indicating if any axis should be drawn in logarithmic scale, as in plot.default.

frame.plot

logical, indicating if a ‘frame’ (box) should be drawn; defaults to TRUE, unless axes = FALSE is specified.

horizontal

logical indicating if the boxplots should be horizontal; default FALSE means vertical boxes.

add

logical, if true add boxplot to current plot.

numeric vector giving the locations where the boxplots should be drawn, particularly when add = TRUE; defaults to 1:n where n is the number of boxes.

show.names

Set to TRUE or FALSE to override the defaults on whether an x-axis label is printed for each group.

pars,...

graphical parameters (etc) can be passed as arguments to this function, either as a list (pars) or normally(...), see the following. (Those in ... take precedence over those in pars.)

Currently, yaxs and ylim are used ‘along the boxplot’, i.e., vertically, when horizontal is false, and xlim horizontally. xaxt, yaxt, las, cex.axis, and col.axis are passed to axis, and main, cex.main, col.main, sub, cex.sub, col.sub, xlab, ylab, cex.lab, and col.lab are passed to title.

In addition, axes is accepted (see plot.window), with default TRUE.

The following arguments (or pars components) allow further customization of the boxplot graphics. Their defaults are typically determined from the non-prefixed version (e.g., boxlty from lty), either from the specified argument or pars component or the corresponding par one.

boxwex:: a scale factor to be applied to all boxes. When there are only a few groups, the appearance of the plot can be improved by making the boxes narrower. The default depends on at and typically is $0.8$.

staplewex, outwex:

staple and outlier line width expansion, proportional to box width; both default to 0.5.

boxlty, boxlwd, boxcol, boxfill:

box outline type, width, color, and fill color (which currently defaults to col and will in future default to par("bg")).

medlty, medlwd, medpch, medcex, medcol, medbg:

median line type, line width, point character, point size expansion, color, and background color. The default medpch = NA suppresses the point, and medlty = "blank" does so for the line. Note thatmedlwd defaults to $3x$ the default lwd.

whisklty, whisklwd, whiskcol:

whisker line type (default: "dashed"), width, and color.

staplelty, staplelwd, staplecol:

staple (= end of whisker) line type, width, and color.

outlty, outlwd, outpch, outcex, outcol, outbg:

outlier line type, line width, point character, point size expansion, color, and background color. The default outlty = "blank" suppresses the lines and outpch = NA suppresses points.

Value

An invisible vector, actually identical to the at argument, with the coordinates ("x" if horizontal is false, "y" otherwise) of box centers, useful for adding to the plot.

References

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

Examples

Run this code

require(stats)
set.seed(753)
(bx.p <- boxplot(split(rt(100, 4), gl(5, 20))))
op <- par(mfrow =  c(2, 2))
bxp(bx.p, xaxt = "n")
bxp(bx.p, notch = TRUE, axes = FALSE, pch = 4, boxfill = 1:5)
bxp(bx.p, notch = TRUE, boxfill = "lightblue", frame = FALSE,
    outl = FALSE, main = "bxp(*, frame= FALSE, outl= FALSE)")
bxp(bx.p, notch = TRUE, boxfill = "lightblue", border = 2:6,
    ylim = c(-4,4), pch = 22, bg = "green", log = "x",
    main = "... log = 'x', ylim = *")
par(op)
op <- par(mfrow = c(1, 2))

## single group -- no label
boxplot (weight ~ group, data = PlantGrowth, subset = group == "ctrl")
## with label
bx <- boxplot(weight ~ group, data = PlantGrowth,
              subset = group == "ctrl", plot = FALSE)
bxp(bx, show.names=TRUE)
par(op)


z <- split(rnorm(1000), rpois(1000, 2.2))
boxplot(z, whisklty = 3, main = "boxplot(z, whisklty = 3)")

## Colour support similar to plot.default:
op <- par(mfrow = 1:2, bg = "light gray", fg = "midnight blue")
boxplot(z,   col.axis = "skyblue3", main = "boxplot(*, col.axis=..,main=..)")
plot(z[[1]], col.axis = "skyblue3", main =    "plot(*, col.axis=..,main=..)")
mtext("par(bg=\"light gray\", fg=\"midnight blue\")",
      outer = TRUE, line = -1.2)
par(op)

## Mimic S-Plus:
splus <- list(boxwex = 0.4, staplewex = 1, outwex = 1, boxfill = "grey40",
              medlwd = 3, medcol = "white", whisklty = 3, outlty = 1, outpch = NA)
boxplot(z, pars = splus)
## Recycled and "sweeping" parameters
op <- par(mfrow = c(1,2))
 boxplot(z, border = 1:5, lty = 3, medlty = 1, medlwd = 2.5)
 boxplot(z, boxfill = 1:3, pch = 1:5, lwd = 1.5, medcol = "white")
par(op)
## too many possibilities
boxplot(z, boxfill = "light gray", outpch = 21:25, outlty = 2,
        bg = "pink", lwd = 2,
        medcol = "dark blue", medcex = 2, medpch = 20)