How to use `histfunc`

I’m trying to create some sort of weighted histogram to visualize cumulative error. The ’sum' for histfunc should theoretically work for this use case, but it doesn’t seem to have any effect on the histogram plot:

Am I using it wrong, or is this a bug?

Answered in

Please don’t open a github issue and a forum post for the same questions. It’s annoying.

I’m sorry for the double posting. I really wasn’t sure where this belongs, as I’ve searched both issues and forum and found no mentions of histfunc anywhere.