You are really making me curious
Already thanks in advance for putting the effort in this
Probably the B0 variable en the OG variable will be on the same axis parallel to the ADF axis whereas both the B1 and FG axis are curves of one of them is curved.
For your information the physical story behind this:
Brix and Gravity both depend on sugar concentration and alcohol concentration. During beer production sugar is converted to alcohol meaning that sugar concentration decreases and alcohol concentration increases. At time=0 there is no alcohol present and the sugar concentration is maximal so Brix=B0 and gravity=OG. Therefor at time=0 there is a simple relation between B0 and OG (making the scales coincide).
The conversion between sugar and alcohol is a simple linear relation: one part of sugar (in moles) is converted to two parts alcohol (in moles). In weight percentages there is a conversion factor between those two.
Brix value depends on both sugar concentration and alcohol concentration but has a difference sensitivity to both of them. There will a relation like Brix=a*sugarconcentration+b*alcoholconcentration.
Gravity also depends on both sugar concentration and alcohol concentration and has a difference sensitivity to both of them. There will a relation like Gravity=c*sugarconcentration+d*alcoholconcentration.
Therefor I expect the B1 and FG lines to be differently orientated (curved) but may be parallel to each other due to differenr constants a, b, c and d.
The nice thing is that when one knows OG or B0 and the ADF, the alcohol content is also known independent on using either the OG-FG-ADF nomogram or the B0-B1-ADF nomogram. The alcohol content should be some kind of connecting line.
The alcohol content by volume (ABV) is given by: (in terms of OG and FG): ABV = 486.8693 * (OG - FG) / (4749.804 - FG)