Well, unless I'm missing something it seems fairly straight forward.
The lines on the graph all appear to be straight so the arms must therefore all be constant. The fuel appears to have an arm of some 38.75", baggage area 1 some 64" and baggage area 2 some 85.7". And if I read the graph correctly, the occupants and the fuel have the same arm.
As weight and balance seems to be a bit of a black art to many, the manufacturer has probably chosen to present the data in this fashion to simplify the issue as much as possible, thus increasing the chances that pilots will do w&b calculations at all whilst at the same time reducing the chances of gross errors.
Even though the arm data might not appear in the AFM, the author of the graph will have obtained it from the manufacturer.
For simple aircraft such as this the fuel arm will generally be constant (a rectangular fuel tank fitted in a straight wing). However on swept wing aircraft which may also have belly tanks and then some sort of automatic scheduling system for the order in which tanks are used, a graph of the fuel arm can be a very convoluted thing.
Hope it helps.
MT