How to Create a Simple, Yet Effective, Line Plot

How to answer the question, “Do I have an obvious trend?”

Last time around, I discussed how to plot a simple, yet effective, scatterplot. In this third installment of my “how-to series,” I want to explore another common chart type: the line plot.

A line (a.k.a. time series) plot is an intuitive way to determine the trend of observations over time. Typically, our observations are counted and displayed on the Y-axis, while time (e.g., hours, days, weeks, years) is displayed on the X-axis.

Figure 1: An example of one of the first time-based charts (Source: A New Chart of History (1769)).

Timeline charts were created in the mid-1760s by Joseph Priestley. In Figure 1 above, you can see how Priestley plotted the rise and fall of empires over time (Y-axis) and the countries that they colonised (as categorical data on the X-axis). The value of such a chart is clear to see: the rise and fall of the Roman Empire.

Figure 2: Playfair’s trade-balance, time-series chart, published in his Commercial and Political Atlas, 1786.

In 1786 William Playfair, a Scottish engineer, politician, and secret agent developed what we consider today to be the first time series plot. Figure 2 above illustrates a time series plot from his book The Commercial and Political Atlas and Statistical Breviary. Looking at the…