2.2. Diagrams (charts, graphs and plots)

The terms diagram, chart, graph and plot often overlap in their definition and on this site I will not make any distinction between them. Diagrams are used to visualize data. There is a very long list of types of diagrams that can be used; the most common ones will be discussed on this site.

The purpose of a diagram should be to describe the data in a clear and efficient way. The first question to ask is, is a diagram necessary? Figure 2 shows an example of a diagram from Wittke-Thompson, Pluzhnikov & Cox (2005).

Figure 2. Example of a non-needed diagram. Reprinted from “Rational Inferences about Departures from Hardy-Weinberg Equilibrium,” by J.K. Wittke-Thompson, A. Pluzhnikov and N.J. Cox, The American Journal of Human Genetics, 76(6), p. 971.

The problem with this chart is that it should not exist. It shows that all values were zero. The author could have saved the space (it takes up about half the page) by simply writing that in text.

Ones you determined that a diagram can be of additional interest the next decision is to determine which type of diagram to use.

• If you have one discrete variable, you could either use a circle diagram or a bar chart.
• If you have one continuous variable, you could use a histogram or frequency polygon
• If you have two discrete variables, you could either use a compound or clustered bar-chart
• If you have two continuous variables you could use a scatterplot, or line

>>Next entry: Circle diagram (a.k.a. Pie chart)

References
Wittke-Thompson, J. K., Pluzhnikov, A., & Cox, N. J. (2005). Rational Inferences about Departures from Hardy-Weinberg Equilibrium. The American Journal of Human Genetics, 76(6), 967–986. doi:10.1086/430507