Great stuff! Looks like the whole dataset was imported 2 days back. If you can stream it daily, I can use table decorator and avoid processing all the records.

There are items in there that are deleted/[dead] on HN (and not very recent). How come?

You can see dead things on HN if you turn on "showdead" in your account settings.

What is the "comment_ranking" data you mention in the notebook?

Hacker News chose to hide comment scores some time ago, but I still wanted to find a way to rank comments. The good news is that the API gives you a "kids" column that ranks comments in the order they should be displayed - that's how I can find what's the top comment for each post (as shown in the linked notebook).

Is that column independent of "gravity"?

I guess it incorporates gravity - it's whichever way Hacker News tells its clients to rank the comments by.