

   OOrrddiinnaatteess ffoorr PPrroobbaabbiilliittyy PPlloottttiinngg

        ppoints(n, a = ifelse(n <= 10, 3/8, 1/2))

   DDeessccrriippttiioonn::

        Generates the sequence of ``probability'' points `(1:m
        - a)/(m + (1-a)-a)' where `m' is either `n', if
        `length(n)==1', or `length(n)'.

   DDeettaaiillss::

        If 0 < a < 1, the resulting values are within (0,1)
        (excluding boundaries).  In any case, the resulting
        sequence is symmetric in [0,1], i.e., `p + rev(p) ==
        1'.

        `ppoints()' is used in `qqplot' and `qqnorm' to gener-
        ate the set of probabilities at which to evaluate the
        inverse distribution.

   SSeeee AAllssoo::

        `qqplot', `qqnorm'.

   EExxaammpplleess::

        ppoints(4) # the same as  ppoints(1:4)
        ppoints(10)
        ppoints(10, a=1/2)

