Seasonal fluctuations of antibiotic sales are shown in the left column.

ambulatory (dark grey, left axis) and hospitals (light grey, right axis). The data are normalized to the annual mean. On the right side of the figure, two columns of plots show periodograms for ambulatory sales and hospital usage (left to right, respectively). The peak in the ambulatory-sales periodogram occurs at 0.08 month−1, i.e., a period of 12 months for these sales, indicated by the dashes.