Playfair's Data on Wages and the Price of Wheat

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.

Usage

data(Wheat)
data(Wheat.monarchs)

Format

Wheat A data frame with 53 observations on the following 3 variables.

Year

Year, in intervals of 5 from 1565 to 1821: a numeric vector

Wheat

Price of Wheat (Shillings/Quarter bushel): a numeric vector

Wages

Weekly wage (Shillings): a numeric vector

Wheat.monarchs A data frame with 12 observations on the following 4 variables.

name

Reigning monarch, a factor with levels Anne Charles I Charles II Cromwell Elizabeth George I George II George III George IV James I James II W&M

start

Starting year of reign, a numeric vector

end

Starting year of reign, a numeric vector

commonwealth

A binary variable indicating the period of the Commonwealth under Cromwell

Source

Playfair, W. (1821). Letter on our Agricultural Distresses, Their Causes and Remedies. London: W. Sams, 1821

Data values: originally digitized from http://www.math.yorku.ca/SCS/Gallery/images/playfair-wheat1.gif now taken from http://mbostock.github.com/protovis/ex/wheat.js

References

Friendly, M. & Denis, D. (2005). The early origins and development of the scatterplot Journal of the History of the Behavioral Sciences, 41, 103-130.

Examples

data(Wheat)

data(Wheat)

# ------------------------------------
# Playfair's graph, largely reproduced
# ------------------------------------

# convenience function to fill area under a curve down to a minimum value
fillpoly <- function(x,y, low=min(y),  ...) {
    n <- length(x)
    polygon( c(x, x[n], x[1]), c(y, low, low), ...)
}

# For best results, this graph should be viewed with width ~ 2 * height
# Note use of type='s' to plot a step function for Wheat
#   and panel.first to provide a background grid()
#     The curve for Wages is plotted after the polygon below it is filled
with(Wheat, {
    plot(Year, Wheat, type="s", ylim=c(0,105), 
        ylab="Price of the Quarter of Wheat (shillings)", 
        panel.first=grid(col=gray(.9), lty=1))
    fillpoly(Year, Wages, low=0, col="lightskyblue", border=NA)
    lines(Year, Wages, lwd=3, col="red")
    })


# add some annotations
text(1625,10, "Weekly wages of a good mechanic", cex=0.8, srt=3, col="red")

# cartouche
text(1650, 85, "Chart", cex=2, font=2)
text(1650, 70, 
	paste("Shewing at One View", 
        "The Price of the Quarter of Wheat", 
        "& Wages of Labor by the Week", 
        "from the Year 1565 to 1821",
        "by William Playfair",
        sep="\n"), font=3)

# add the time series bars to show reigning monarchs
# distinguish Cromwell visually, as Playfair did
with(Wheat.monarchs, {
	y <- ifelse( !commonwealth & (!seq_along(start) %% 2), 102, 104)
	segments(start, y, end, y, col="black", lwd=7, lend=1)
	segments(start, y, end, y, col=ifelse(commonwealth, "white", NA), lwd=4, lend=1)
	text((start+end)/2, y-2, name, cex=0.5)
	})

# -----------------------------------------
# plot the labor cost of a quarter of wheat
# -----------------------------------------
Wheat1 <- within(na.omit(Wheat), {Labor=Wheat/Wages})
with(Wheat1, {
	plot(Year, Labor, type='b', pch=16, cex=1.5, lwd=1.5, 
	     ylab="Labor cost of a Quarter of Wheat (weeks)",
	     ylim=c(1,12.5));
	lines(lowess(Year, Labor), col="red", lwd=2)
	})
	
# cartouche
text(1740, 10, "Chart", cex=2, font=2)
text(1740, 8.5, 
	paste("Shewing at One View", 
        "The Work Required to Purchase", 
        "One Quarter of Wheat", 
        sep="\n"), cex=1.5, font=3)

with(Wheat.monarchs, {
	y <- ifelse( !commonwealth & (!seq_along(start) %% 2), 12.3, 12.5)
	segments(start, y, end, y, col="black", lwd=7, lend=1)
	segments(start, y, end, y, col=ifelse(commonwealth, "white", NA), lwd=4, lend=1)
	text((start+end)/2, y-0.2, name, cex=0.5)
	})