Howto correctly analyze knock?

and how do you prevent knock (except ignition retard) from the engine building side? See the attached image, blue graph is rpm, yellow knock for cylinder 3, third, fourth, fith, sixth gear, wot. Knock increases with rpm, just cylinder 3. What would you do do flatten the graph? What causes this behaviour, please explain. Thank you!

The first point which is critical is to make sure that your knock detection strategy is correctly calibrated in the ECU and that the ECU can properly distinguish knock from background engine noise. This should be done with audio knock detection equipment so that you can audibly hear the engine knock. There is a webinar about this here on the Link platform but the techniques work on any platform: https://www.hpacademy.com/previous-webinars/049-setting-up-knock-control-link-g4-vipec/

It's also common to see the background noise level from the knock control system rise with engine rpm. This is why there is a threshold which you can adjust based on engine speed. It's also important that you have this threshold is correct. You may also find that the background levels from each cylinder is different which you need to adjust with the individual cylinder gains.

Assuming that the knock detection is working and the ECU is correctly detecting knock then your options are to retard the timing, richen the mixture (you can try this on a specific cylinder), or both. It's not uncommon to find an engine will start knocking on 1-2 cylinders before the others. From an engine building perspective you can lower the compression ratio but it's also an advantage to make sure that there are no sharp or rough edges in the combustion chamber or on the piston.