Again, these are nice data, but cannot stand alone in presentation without more perspective. Taken with your previous graph, it is clear that the rode does not have to be on the bottom to retain much of the holding power of the anchor. So the question becomes, how much rode at which depths is required to maintain an angle at or below that which preserves (say) 80% of the holding power of the anchor.
Again, I suspect the story is more fleshed out in the article context than I am seeing in two graphs.
Yes, that is a good way of stating the question. For any give set of conditions (wind and bottom type) and depth, how much scope (length) do I need to maintain enough holding capacity? In shallow water, over soft mud, with a less efficient anchor and thunderstorm coming, you probably need 10:1 and a long snubber, even with chain. You only have 70' of chain out, and with 1000 pounds of tension, it's going to get pretty darn straight. In 20 feet, over sticky mud, with just a little blustery weather and an oversized NG anchor, 5:1 and a short snubber is more than enough. The chain might come off the bottom, but probably not, and not enough to matter.
It's not something you describe in a few posts. But if you follow the math, the folks that say all-chain, use conservatively sized anchors, and anchor in deeper water, and the people who say rope can work and use pivoting fluke anchors in good bottoms, are both right. The people that say you need a long snubber are right in some conditions, and the folks that claim 6' is enough are right in some conditions. Obviously, members of both groups have practiced their craft for many years. In fact, my anchoring practices with my trimaran (rope) are quite different from those when I had the cruising cat (all chain). The math is different and both are correct.