an R model formula, of the form ~ variable to estimate the
unconditional density of variable, or variable ~ factor to estimate the
density of variable within each level of factor.
data
an optional data frame containing the data.
main
main title of plot
xlab
label for the horizontal-axis; defaults to the name of the
variable x.
ylab
label for the vertical axis; defaults to "Density".
...
arguments to be passed to plot
Value
This function return NULL invisibly and draw graphs.