FWIW, here's a spectrogram I coded in Matlab using my data I posted yesterday. The two white lines show the -3 and -6dB values, as requested by another individual. Per the title, the response is normalized to the on-axis response, so you can see some areas where the off-axis response has a slightly higher SPL value than the on-axis response (3-4kHz, for example). I prefer this way of viewing spectrogram data because it tells me how the response varies relative to the on-axis response. But I can easily change the program to deliver absolute SPL levels as well. I prefer to write my own scripts because it puts me in control of how I present the data as opposed to using bits and pieces of others' software (such as aRTA or VirtuixCAD; both great programs, just not how I exactly want to present the data).