If i understood your request correctly: dB is labelled top left, hertz is labelled bottom right. It s 'дб' and 'гц' handwritten. Accordingly, 0dB in this diagram is the lowest limiting line.
@ivan zlax In terms of simply reading the diagram, yes. Yet, when using the data to create a calibration file, then one would declare a particular level to be 0dB (here probably approx. 26 dB ^= 0dB) and then write out the data set relative to this new 0dB line. This results in a logarhythmical sequency of levels per frequency (20Hz to 20000Hz), levels deviating up or down from 0 dB, and can as such be used later to recalibrate/equalize a recorded signal to have a flat frequency response.
This is used in calibration of listebning setups, which is, what i am doing with my oktavas here at the moment. Besides i love them for recording of course, and then i don't care if the have a flat/neutral frequency response, i love them for their special sound.