plot the density for multiple samples
plotden(data, lv, index = NULL, name = NULL, lwd = 1, ...)
data row as peaks and column as samples
group information
index for selected peaks
name on the figure for samples
the line width for density plot, default 1
parameters for `plot` function
data(list)
plotden(list$data, lv = as.character(list$group$sample_group),ylim = c(0,1))