The testing mechanism for octane measurement is indeed a variable compression ratio engine. All of the other parameters that I mentioned, however, remain constant. For THAT engine you could easily draw a chart, but it would be good only for THAT engine with THOSE particular mixture and timing parameters.

In the real world, all engine designs have different combustion chamber and valve configurations as well as different fuel and ignition management systems. That means multiple, interacting parameters.

Have a great day,