Percent of Total in Histogram Query

Hello,

I'd like to write a histogram-like query that shows these columns:
  - x-value from 0 to k.
  - number of rows with that x-value.
  - number of rows seen so far (i.e. with the current x-value or less).
  - % of total rows seen so far.

The following query works for the first three columns, but the last column gives me an error:

        SELECT c,
                COUNT(*) AS items_count,
                SUM(COUNT(*)) OVER (ORDER BY c) AS total_items_count,
                SUM(COUNT(*)) OVER (ORDER BY c) / SUM(COUNT(*)) AS total_items_perc
        FROM    (SELECT p.id, COUNT(*) c
                 FROM   parent p, child ch
                 WHERE  p.id = ch.parent_id
                 GROUP BY p.id
                ) x
        GROUP BY x.c
        ORDER BY x.c

Including that fourth SELECT column gives me this error:

    ERROR:  aggregate function calls cannot be nested

Is there any way to get % of total in a query like this?

Thanks,
Paul

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Pulchritudo splendor veritatis.