Draws line segments or rectangles.  Mainly intended to be used in
  conjunction with the segplot function.
prepanel.segplot(x, y, z, subscripts, horizontal = TRUE, ...)panel.segplot(x, y, z, level = NULL, subscripts, 
              at,
              draw.bands = is.factor(z),
              col, alpha, 
              lty, lwd,
              border,
              col.symbol = col, alpha.symbol = alpha,
              col.regions = regions$col,
              band.height = 0.6,
              horizontal = TRUE, 
              ...,
              segments.fun = panel.segments,
              centers = NULL,
              pch = 16)
Vectors corresponding to x1, x2 and
    y respectively in the segplot formula.  The names are
    different for compatibility with panel.levelplot.  These are
    all the original vectors in data, not subsetted for
    particular panels.
optional vector controlling color of segments
optional vector of ‘centers’ of the segments.
    If specified, points will be plotted at these y-locations.
plotting character used for centers.
integer subscript to be used as an indexing vector
    for x, y, z and level, giving the packet
    for the current panel.
logical, whether the segments are to be drawn horizontally (the default) or vertically. This essentially swaps the role of the x- and y-axes in each panel.
values of level where color code changes
logical, whether to draw rectangles instead of lines
Graphical parameters for the
    segment.  Defaults to parameter settings for "plot.line" or
    "plot.polygon" for segments and rectangles respectively.
    col is overridden by col.regions if level is
    not null.
Graphical parameters for the point if
    centers are plotted.  Defaults to the corresponding
    parameters for the segment.
vector of colors as in levelplot
height of rectangles (applicable if
    draw.bands is TRUE
Other arguments, passed on to panel.rect (when
    draw.bands=TRUE), segments.fun (otherwise),
    panel.points (if centers is not NULL), etc. as
    appropriate.
function used to plot segments when
    draw.bands is FALSE.  The default is to use
    panel.segments, but panel.arrows is a
    useful alternative (arguments to segments.fun can be provided
    via the … argument, see example for
    segplot).
For prepanel.segplot a list with components xlim and
  ylim.