[Occasionally6]

Those are curves, not a map. They are developed at a single throttle opening i.e. wide open, meaning the inlet manifold pressure is very close to atmospheric. That's the maximum the engine will do at a given rpm. The single curves are not a whole lot of use because you don't (usually) drive with WOT.

What is needed for a BSFC map is the BSFC figures corresponding to power outputs at a range of manifold pressures and rpm. You might generate that by holding the inlet manifold pressure constant, at a number of different pressures, and generating similar curves to those above for each pressure. They can be then be displayed together in the form of 3D maps; manifold pressure vs rpm vs BSFC and manifold pressure vs rpm vs torque. Power is, simplistically, torque X rpm.

Because a given power output can be obtained at more than one combination of manifold pressure (~ throttle opening) and rpm, that map allows you to pick the most efficient combination from the load vs rpm points at which the power you want/need can be generated.