boxplot               package:graphics               R Documentation

Box Plots

Description:

     Produce box-and-whisker plot(s) of the given (grouped) values.

Usage:

     boxplot(x, ...)

     ## S3 method for class 'formula':
     boxplot(formula, data = NULL, ..., subset, na.action = NULL)

     ## Default S3 method:
     boxplot(x, ..., range = 1.5, width = NULL, varwidth = FALSE,
             notch = FALSE, outline = TRUE, names, plot = TRUE,
             border = par("fg"), col = NULL, log = "",
             pars = list(boxwex = 0.8, staplewex = 0.5, outwex = 0.5),
             horizontal = FALSE, add = FALSE, at = NULL)

Arguments:

 formula: a formula, such as 'y ~ grp', where 'y' is a numeric vector
          of data values to be split into groups according to the
          grouping variable 'grp' (usually a factor).

    data: a data.frame (or list) from which the variables in 'formula'
          should be taken.

  subset: an optional vector specifying a subset of observations to be
          used for plotting.

na.action: a function which indicates what should happen when the data
          contain 'NA's.  The default is to ignore missing values in
          either the response or the group.

       x: for specifying data from which the boxplots are to be
          produced. Either a numeric vector, or a single list
          containing such vectors. Additional unnamed arguments specify
          further data as separate vectors (each corresponding to a
          component boxplot). 'NA's are allowed in the data.

     ...: For the 'formula' method, named arguments to be passed to the
          default method.

          For the default method, unnamed arguments are additional data
          vectors (unless 'x' is a list when they are ignored), and
          named arguments are arguments and graphical parameters to be
          passed to 'bxp' in addition to the ones given by argument
          'pars' (and override those in 'pars'). 

   range: this determines how far the plot whiskers extend out from the
          box.  If 'range' is positive, the whiskers extend to the most
          extreme data point which is no more than 'range' times the
          interquartile range from the box. A value of zero causes the
          whiskers to extend to the data extremes.

   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.

   notch: if 'notch' is 'TRUE', a notch is drawn in each side of the
          boxes.  If the notches of two plots do not overlap this is
          'strong evidence' that the two medians differ (Chambers _et
          al._, 1983, p. 62).  See 'boxplot.stats' for the calculations
          used.

 outline: if 'outline' is not true, the outliers are not drawn (as
          points whereas S+ uses lines).

   names: group labels which will be printed under each boxplot. Can be
          a character vector or an expression (see plotmath).

  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.

staplewex: staple line width expansion, proportional to box width.

  outwex: outlier line width expansion, proportional to box width.

    plot: if 'TRUE' (the default) then a boxplot is produced.  If not,
          the summaries which the boxplots are based on are returned.

  border: an optional vector of colors for the outlines of the
          boxplots.  The values in 'border' are recycled if the length
          of 'border' is less than the number of plots.

     col: if 'col' is non-null it is assumed to contain colors to be
          used to colour the bodies of the box plots. By default they
          are in the background colour.

     log: character indicating if x or y or both coordinates should be
          plotted in log scale.

    pars: a list of (potentially many) more graphical parameters, e.g.,
          'boxwex' or 'outpch'; these are passed to 'bxp' (if 'plot' is
          true); for details, see there.

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

     add: logical, if true _add_ boxplot to current plot.

      at: 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.

Details:

     The generic function 'boxplot' currently has a default method
     ('boxplot.default') and a formula interface ('boxplot.formula').

     If multiple groups are supplied either as multiple arguments or
     via a formula, parallel boxplots will be plotted, in the order of
     the arguments or the order of the levels of the factor (see
     'factor').

     Missing values are ignored when forming boxplots.

Value:

     List with the following components: 

   stats: a matrix, each column contains the extreme of the lower
          whisker, the lower hinge, the median, the upper hinge and the
          extreme of the upper whisker for one group/plot.  If all the
          inputs have the same class attribute, so will this component.

       n: a vector with the number of observations in each group.

    conf: a matrix where each column contains the lower and upper
          extremes of the notch.

     out: the values of any data points which lie beyond the extremes
          of the whiskers.

   group: a vector of the same length as 'out' whose elements indicate
          to which group the outlier belongs.

   names: a vector of names for the groups.

References:

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

     Chambers, J. M., Cleveland, W. S., Kleiner, B. and Tukey, P. A.
     (1983) _Graphical Methods for Data Analysis._  Wadsworth &
     Brooks/Cole.

     Murrell, P. (2005) _R Graphics_. Chapman & Hall/CRC Press.

     See also 'boxplot.stats'.

See Also:

     'boxplot.stats' which does the computation, 'bxp' for the plotting
     and more examples; and 'stripchart' for an alternative (with small
     data sets).

Examples:

     ## boxplot on a formula:
     boxplot(count ~ spray, data = InsectSprays, col = "lightgray")
     # *add* notches (somewhat funny here):
     boxplot(count ~ spray, data = InsectSprays,
             notch = TRUE, add = TRUE, col = "blue")

     boxplot(decrease ~ treatment, data = OrchardSprays,
             log = "y", col = "bisque")

     rb <- boxplot(decrease ~ treatment, data = OrchardSprays, col="bisque")
     title("Comparing boxplot()s and non-robust mean +/- SD")

     mn.t <- tapply(OrchardSprays$decrease, OrchardSprays$treatment, mean)
     sd.t <- tapply(OrchardSprays$decrease, OrchardSprays$treatment, sd)
     xi <- 0.3 + seq(rb$n)
     points(xi, mn.t, col = "orange", pch = 18)
     arrows(xi, mn.t - sd.t, xi, mn.t + sd.t,
            code = 3, col = "pink", angle = 75, length = .1)

     ## boxplot on a matrix:
     mat <- cbind(Uni05 = (1:100)/21, Norm = rnorm(100),
                  `5T` = rt(100, df = 5), Gam2 = rgamma(100, shape = 2))
     boxplot(as.data.frame(mat),
             main = "boxplot(as.data.frame(mat), main = ...)")
     par(las=1)# all axis labels horizontal
     boxplot(as.data.frame(mat), main = "boxplot(*, horizontal = TRUE)",
             horizontal = TRUE)

     ## Using 'at = ' and adding boxplots -- example idea by Roger Bivand :

     boxplot(len ~ dose, data = ToothGrowth,
             boxwex = 0.25, at = 1:3 - 0.2,
             subset = supp == "VC", col = "yellow",
             main = "Guinea Pigs' Tooth Growth",
             xlab = "Vitamin C dose mg",
             ylab = "tooth length",
             xlim = c(0.5, 3.5), ylim = c(0, 35), yaxs = "i")
     boxplot(len ~ dose, data = ToothGrowth, add = TRUE,
             boxwex = 0.25, at = 1:3 + 0.2,
             subset = supp == "OJ", col = "orange")
     legend(2, 9, c("Ascorbic acid", "Orange juice"),
            fill = c("yellow", "orange"))

     ## more examples in  help(bxp)

