Ah, sorry! Such an oversight! (You get so involved in a project you think it's all natural...)

The histograms are:

For Reddit, they are of a headline's votes, of a headline's number of comments, and of a comment's votes.

For Delicious, they are of the numbers of users that have bookmarked a link, or applied a tag.

For Digg, of a headline's diggs, of a headline's number of comments, and of a comment's diggs.

For Hacker News, of a headline's upvotes, of a headline's number of comments, and of a comment's upvotes.

For StackOverflow, of a question's views, of an answer's votes, of the number of users that have applied a tag, of a user's reputation score.

It's hopefully all obvious once you install the script.

Adding larger versions of the screenshots in a sec..

A histogram shows a distribution of values. This just looks like a bar graph. Still useful, but I don't see how it's a histogram.

Ah, I see, I didn't even know there was a difference, thanks for pointing it out. I've always like the word "bar graph" more (it's more transparent), so bar graphs they are now!

A histogram of votes/diggs/etc where each story title is a bar in the histogram. I didn't get it at first but if you look at the delicious pic you can see clearly how it works with the tags sidebar.