This is the only paper I've found regarding a DC-9's takeoff prep
Then do make sure that the model used and the one you are trying to model are the same - the numbers do differ between models.
Well, in order to actually fly you may still fly even slightly outside the graph (but with greater difficulty) with whatever stab,
While there is some fat in the MAC grid to stab setting relationship, the aim for the operator and crew is to set the stab as accurately as is reasonably achievable. The only residual errors should be associated with the (intentional) errors internal to the design of the loading system graphs (and, of course, any unfortunate and unintended loading errors on the day).
you normally set your stab according to the CG's position which lies at "X" MAC percentage
Correct and that is what is occuring in your graph - it's just that the designer didn't bother to indicate the MAC values which, at the end of the day, are only an intermediate step of no particular interest to the crew.
so I still don't know why do they use only weights as reference instead of CG position in order to set up your stab!
You appear not to be listening to the discussion. The graph fan grid IS the MAC grid relevant to the aircraft. It's just not scaled explicitly and there is absolutely no reason for it to be so scaled. With the relevant model AFM/WCM data, one, quite easily, could reverse engineer the thing and include the MAC values. Not sure why you would bother doing so but you could, should you so desire.
There is no C of G here, yes, so i was wondering if anyone might know how to extract it from this graph!
Again, the fan grid IS the CG (MAC) grid. With the AFM/WCM data, a very simple reverse engineering exercise to determine the CG values. Unfortunately, the only data I can find doesn't match the graph's data so we hit a barrier there ...
so if the horizontal axis may indeed refer to a longitudinal position taking the datum as reference, that may be converted into a CG position and subsequently into a MAC % position.
This NEVER can be the case. In a trim sheet, the scales will ALWAYS be gross weight by moment or IU (which are just different ways of looking at the same thing). This causes lots of heartache for folks as we see two styles of chart. In the limitations section the data, typically, is presented as weight by CG. In the loading system or weight and balance section, the data will be presented, usually, as weight by moment/IU or, for the inconvenience of having an extra bit of work, weight by CG. However, a trim sheet style of graph can ONLY EVER be weight by moment/IU - anything else is ALWAYS WRONG. If you want the CG values, that involves some extra calculation, in this case, a reverse engineering exercise.
something like this would've been very good for what I need
The usual presentation is to run the MAC fan lines up to the top of the graph and then overplot with separate stab setting lines to suit each flap setting.
but that chart which misses the C of G reference is all I have
As I suggested earlier, the TCDS data for the referenced model doesn't quite match the graph very well. If you can find the data upon which the graph was designed, we can reverse engineer the data you seek .. very quickly.
I think it would help your understanding, greatly, if you had a look through the thread hyperlinked at post #7. That thread provides a very comprehensive overview of trimsheet design problems.