That person is sampling the top posts instead of sampling all posts. I don't think that's predictive. Particularly anything where you sample extremes (maximum, or top N) is susceptible to extreme fluctuations.

He wants traffic, metrics such as "# of votes per post submitted", "probability of hitting the top page", etc. To model a successful post you have to also model an unsuccessful post.

My take is that HN always has activity because the sun is shining somewhere and that addicts are using it at every hour (but few from midnight to 5pm)

My favorite time to post for maximum traffic is 7am EST or so because at that point Europe is in full swing and the U.S. east coast is waking up. If you catch a break from the Europeans then you get a swelling audience across the U.S. and can hang near the top for a long time.

The highest karma user on HN is in Brazil, which might be very good to work the Euro-American audience.

You forgot the most important thing here ortherwise your graph become useless: Timezone.