The horizontal axis of an NMR spectrum is "chemical shift". A standard environment such as tetramethylsilane is chosen as the zero of chemical shift. The frequency at which resonance occurs for other environments is then measured relative to the frequency at which resonance of the standard environment occurs e.g. a particular environment might resonate at a frequency 1ppm lower than standard environment.
Why can't we measure the frequencies at which resonance occurs absolutely, not relative to a standard environmnent? Then we would have a horizontal axis of frequency instead of chemical shift.