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.