There are a couple of issues here.
The vertical scale we normally use brackets the standard 75dB target level used with REW. The scale used is from 45dB to 105dB. This allows a reasonable min/max of 60dB centered on 75dB. Consistency is important for comparisons, so we like to stick to that scale.
The horizontal scale we normally use is 15Hz to 200Hz. The 15Hz is optional to 10Hz for those with an IB etc, but anything below that is not meaningful, since most people have a microphone calibration file that ends at 10Hz. Measuring below that would require a fairly accurate mic with an associated cal file.
The 200Hz was chosen to accommodate a standard value (where in association with the recommended vertical scale), an 80Hz crossover target would just fall below 45dB before 200Hz. This also accommodated adding the mains speakers to check for crossover interaction without changing scales.
We have found that the vertical (45dB -105dB) and horizontal (15Hz - 200Hz) scale to be a good overall compromise so that everyone is comparing apples to apples.
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brucek