Description:
Playfair (1821) used a graph, showing parallel time-series of the price of wheat and the typical weekly wage for a "good mechanic" from 1565 to 1821 to argue that working men had never been as well-off in terms of purchasing power as they had become toward the end of this period.
His graph is a classic in the history of data visualization, but commits the sin of showing two non-commensurable Y variables on different axes. Scatterplots of wages vs. price or plots of ratios (e.g., wages/price) are in some ways better, but both of these ideas were unknown in 1821.
In this version, information on the reigns of British monarchs is provided in a separate data.frame, Wheat.monarch.