Default panel function for barchart.
panel.barchart(x, y, box.ratio = 1, box.width,
               horizontal = TRUE,
               origin = NULL, reference = TRUE,
               stack = FALSE,
               groups = NULL, 
               col = if (is.null(groups)) plot.polygon$col
                     else superpose.polygon$col,
               border = if (is.null(groups)) plot.polygon$border
                        else superpose.polygon$border,
               lty = if (is.null(groups)) plot.polygon$lty
                     else superpose.polygon$lty, 
               lwd = if (is.null(groups)) plot.polygon$lwd
                     else superpose.polygon$lwd,
               …, identifier = "barchart")Extent of Bars. By default, bars start at left of panel,
    unless origin is specified, in which case they start there.
Horizontal location of bars. Possibly a factor.
Ratio of bar width to inter-bar space.
Thickness of bars in absolute units; overrides
    box.ratio.  Useful for specifying thickness when the
    categorical variable is not a factor, as use of box.ratio
    alone cannot achieve a thickness greater than 1.
Logical flag. If FALSE, the plot is ‘transposed’ in the sense
    that the behaviours of x and y are switched. x is now the
    ‘factor’.  Interpretation of other arguments change
    accordingly. See documentation of bwplot for a fuller
    explanation.
The origin for the bars.  For grouped displays with
    stack = TRUE, this argument is ignored and the origin set to
    0.  Otherwise, defaults to NULL, in which case bars start at
    the left (or bottom) end of a panel.  This choice is somewhat
    unfortuntate, as it can be misleading, but is the default for
    historical reasons.  For tabular (or similar) data, origin =
      0 is usually more appropriate; if not, one should reconsider the
    use of a bar chart in the first place (dot plots are often a good
    alternative).
Logical, whether a reference line is to be drawn at the origin.
logical, relevant when groups is non-null. If
    FALSE (the default), bars for different values of the
    grouping variable are drawn side by side, otherwise they are
    stacked.
Optional grouping variable.
Graphical parameters for the bars. By default, the trellis parameter
    plot.polygon is used if there is no grouping variable, otherwise
    superpose.polygon is used.  col gives the fill color,
    border the border color, and lty and lwd the
    line type and width of the borders.
Extra arguments will be accepted but ignored.
A character string that is prepended to the names of grobs that are created by this panel function.
A barchart is drawn in the panel. Note that most arguments controlling
  the display can be supplied to the high-level barchart call
  directly.
# NOT RUN {
barchart(yield ~ variety | site, data = barley,
         groups = year, layout = c(1,6), origin = 0,
         ylab = "Barley Yield (bushels/acre)",
         scales = list(x = list(abbreviate = TRUE,
                                minlength = 5)))
# }
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