So it turned out I was looking at the wrong output from my Talys nuclear reactions simulations , of what happens when a uranium-233 nucleus is being bombarded with neutrons with all kinds of energies. (Talys is a program for simulating lots of types of nuclear reactions; it's free, online, and everyone can check it out here, if they want to :) ). When you run a simulation (or 60, like I have) you get a folder with lots of different files, and silly me was looking at the wrong file... "Of course": the file I needed to plot wasn't the "totalxs.tot", but the "rp092234.tot". If I'd read the manual more closely this would have been clear, but it's more than 500 pages. Still on my to do list to read it, though.
Yes, it does feel sort of good when you realise why things aren't working, but it's not like I've solved a great problem, I just happened to be an idiot, looking at the wrong output file (and, of course, it's nice to not be an idiot, but the solving this is'n taking me big leaps forward - I'm just not an idiot right now).
I guess I just have to repeat to myself: Never give up.
no matter how you feel, get up, dress up, show up, and never give up!