When the frequency domain's independent variable is labeled as a fraction of the sampling rate, the values along the horizontal axis always run between $0$ and $0.5$, since discrete data can only contain frequencies between DC ($0$) and half the sampling rate.
But why is the highest frequency pegged at exactly $0.5$ of the sampling rate? Isn't it possible to have a scenario where the highest frequency is less than half the sampling rate?
E.g., consider a speech signal that has been filtered to remove all frequencies above $3.3$ kHz and has been sampled at $10$ kHz. The highest frequency ($3.3$ kHz) is less than $0.5$ of the sampling rate ($5$ kHz). How would the frequency domain of this signal (labeled as a fraction of the sampling rate) look like?