an object of class DZdata, i.e. a list with the
following items:x: a named list of vectors containing the numerical data for each sample
err: an (optional) named list of vectors containing the standard errors of x
metric: either "KS" (for Kolmogorov-Smirnov) or "SH" (for Sircombe Hazelton)
breaks: a vector with the locations of the histogram bin edges
xlabel: a string containing the label to be given to the x-axis on all plots