This is the default panel function for densityplot.
panel.densityplot(x, darg, plot.points = "jitter",
                  ref = FALSE,
                  groups = NULL,
                  weights = NULL,
                  jitter.amount,
                  type, ...,
                  grid = lattice.getOption("default.args")$grid,
                  identifier = "density")
data points for which density is to be estimated
list of arguments to be passed to the density
    function. Typically, this should be a list with zero or more of the
    following components : bw, adjust, kernel,
    window, width, give.Rkern, n,
    from, to, cut, na.rm (see
    density for details)
logical specifying whether or not the data points should be plotted
    along with the estimated density.  Alternatively, a character string
    specifying how the points should be plotted.  Meaningful values are
    "rug", in which case panel.rug is used to plot
    a ‘rug’, and "jitter", in which case the points are
    jittered vertically to better distinguish overlapping points.
logical, whether to draw x-axis
an optional grouping variable.  If present,
    panel.superpose will be used instead to display each
    subgroup
numeric vector of weights for the density
    calculations.  If this is specified, the ... part must
    also include a subscripts argument that matches the weights
    to x.
when plot.points="jitter", the value to use as the
    amount argument to jitter.
type argument used to plot points, if requested.
    This is not expected to be useful, it is available mostly to protect
    a type argument, if specified, from affecting the density
    curve.
extra graphical parameters.  Note that additional
    arguments to panel.rug cannot be passed on through
    panel.densityplot.
A logical flag, character string, or list specifying whether and how
    a background grid should be drawn.  In its general form, grid
    can be a list of arguments to be supplied to
    panel.grid, which is called with those arguments.
    Three shortcuts are available:
TRUE:roughly equivalent to list(h = -1, v = -1)
"h":roughly equivalent to list(h = -1, v = 0)
"v":roughly equivalent to list(h = 0, v = -1)
No grid is drawn if grid = FALSE.
A character string that is prepended to the names of grobs that are created by this panel function.
Deepayan Sarkar Deepayan.Sarkar@R-project.org
densityplot,
  jitter