Full Document: ANSI S3.22: Specification of Hearing Aid Characteristics

Section 6.1, titled Curves, outlines the recommended aspect ratio for graphing. This use to be a requirement. Basically, it says 50dB vertical for a decade of Frequency, which is 15dB vertical / octave. The idea was that by controlling the aspect ratio it would be easier to compare one product to another (back in the days when paper datasheets where how you sold products). Most likely today, the datasheet is done mainly for compliance.