Optimizing Thief-based Xenon, interpreting calculations, & other questions

Your graph is not wrong. From what I see, it tells you how much ATT the next +%something provides in relation to ATT. The thing is, is this information valuable? Does it really help you? If your aim is to keep having the greatest increase in damage, then this graph provides little information.
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First, figure out what you want, then ask the right questions to guide you to what you want, then find the most efficient answers to those question.

Yeah, it will entirely depend on what I want at that moment in time. If I wanted to take the most likely route of upgrading by taking it "step-by-step", then I believe what can be inferred by the information presented in the graph above can be useful, to a certain extent. While it does not provide much information, I feel that it is sufficient to give me a general sense of "what's better than what".

For instance, if I need a new ring and I have the option to pick one of two:

• Ring A: 12% STR, 9% DEX, 9% DEX
  
• Ring B: 6% All Stats, 6% DEX, 6% LUK
  

The graph allows for an analysis of the rings in terms of ATT by simplifying and converting the potentials:

• Ring A: 12% STR, 18% DEX -> 6% STR, 6% DEX (3.929328214 ATT + 4.44619375 ATT)
  
• Ring B: 6% STR, 12% DEX, 12% LUK -> 12% LUK (9.821595329 ATT)
  

Conclusion: I would choose Ring B over Ring A because the ATT-equivalence of its %stats is greater.

In my opinion, a graph like this is useful to me because it provides an intuitive means to compare items with similar stats when the conclusion is not so obvious at first glance. Granted, this method of analysis becomes rather... pointless if I have the tooltips in front of me that tell me exactly the +range increase of each item, or if I perform before & after calculations of ranges and compare their differences.

The idea of comparing relative increases seemed like an effective indicator for which items to pick and choose from (especially when considering the limited stock of certain %items and their wildly divergent prices), which I believe is most important. Although this method fulfills its purpose in the example above, would it not be safe to analogously interpret my findings in this manner? Maybe it's because I consider "%stats-to-ATT" to be synonymous to "%stats-to-actual damage"; can this way of seeing things lead to a misleading conclusion?

Think of it this way. You want to find the highest point in the mountain in the shortest time. How do you do it? Find the steepest path and follow it. Your graph shows you the slope where you are right now, but 2 steps later the slope would have changed. The challenge is to find a way to visualise it, be it in a graph or an equation (which also becomes a graph I guess).

If I were to make something truly "useful" in all aspects, then would I have to create a generalized equation of 8 variables?: (%ATT [which should be constant for the most part, but must be included if there are sources of ATT that are not affected by potential], ATT, %STR, STR, %DEX, DEX, %LUK, LUK)... I imagine it is very difficult to visualize/conceptualize given the amount of variables.

For this reason, I settled with providing just one seemingly useful example of the dozens of tables/graphs that could be written to get a better view of the larger picture. Is this method of analysis by looking at each of the comparisons individually not recommended?

So, for example, are you going to keep doing that graph everytime your potentials change/get better?

I wasn't sure if you were hinting at the amount of labor needed to produce these graphs, but the only numbers that I will have to manually enter are the ever-changing stats/%stats on my equipment. Since the Excel file automatically configures the rest of the calculations for me, it's not too much work.
But yes, it would be my intention to keep myself updated with the graphs as I modify/replace gears.