This is a beautiful example of a cyclical time series.
(Time series means data taken at regular intervals over a period of time; cyclical means recurring in cycles. It is a feature of these graphs that certain conditions repeat themselves every so often, leading to a graph which also tends to be cyclical.)
Q1.What is the general trend in retail sales?
Q2.When, during a cycle, does each violent peak occur? Why? What follows a peak? Why?