This is how I read it:
The vertical axis is BMEP, or brake effective mean pressure - basically torque. An engine with a flat torque curve would be a straight line, but torque in this engine peaks at around 3500RPM and drops off above that (because the engine starts to run out of air, head doesn't flow well), so you have a curved graph.
I'm going to pick 2000RPM as an exampe: Peak BMEP there is ~140 (not sure of units, but it doesn't matter). Peak efficiency is just above 100 (75% of 140), with a highly efficient range of ~80-120 (65-85%). So, ideally you'd be shooting for about 75% load, give or take 10.
Due to how throttle plates work, the throttle opening for 75% load will be much larger at 4000RPM than at 2000RPM, since the engine needs a lot more air. There's no easy way to convert throttle opening to load - for this, you either need a vacuum gauge or an OBD II reader. Luckily, a $5 ELM327 bluetooth dongle + $3 Torque Pro app for Android can show load values:
