How to determine range of wires that can safety fit into wire nut?
You are not wrong.  While Ideal makes great wire nuts, I have to agree those graphs are just junk.  It's not really information that lends itself to a bar graph.
This document is available for US models and gives you the exact info you need:
https://www.idealind.com/content/dam/electrical/assets/WireTermination/WireConnectors/TwistOn/WireNut/UL%20ListedWireCombinations.pdf
Other brands have similar reference materials.  Here is the section for the US equivalent, model 76B:

That's more like it, right?  But it's a lot to put on a label.
The picture in the question is a Canadian version of the wire nut (or would that be a marrette?) and I can't find an equivalent document for their Canadian product numbers.  Ideal's web site was updated a while ago and is half useless.