You are correct that in a pressure-position graph of a sound wave, the normal atmospheric pressure is typically represented as the baseline or zero-pressure reference point. The graph depicts the variations in pressure from this baseline as the sound wave propagates through the medium.
In a sound wave, compressions and rarefactions occur as alternating regions of high pressure (compressions) and low pressure (rarefactions). The compressions correspond to regions where air molecules are densely packed together, leading to an increase in pressure above atmospheric pressure. The rarefactions, on the other hand, correspond to regions where air molecules are spread apart, resulting in a decrease in pressure below atmospheric pressure.
In a pressure-position graph, the representation of a sound wave typically shows the rarefactions as negative pressure values below the baseline, indicating a pressure lower than atmospheric pressure. The compressions, in contrast, are shown as positive pressure values above the baseline, indicating a pressure higher than atmospheric pressure.
So, while the rarefactions are graphically represented as occurring "first" below the baseline in the pressure-position graph, it is important to note that the baseline itself represents the normal atmospheric pressure. The graph captures the fluctuations in pressure relative to this baseline as the sound wave propagates through the medium.