What makes a good scatterplot?

First, scatterplots can be made only with quantitative variables, never with categorical variables. Second, decide if one variable will be used to predict, or explain, another variable. If so, make that the explanatory variable. In other cases, either variable might be explanatory.

Graphs must be clearly labeled so that the variables assigned to each axis are clear. Including units is a good idea, especially if that information is not obvious, or included in nearby text. Each axis must have a clear scale of numbers so that the graph can be accurately interpreted. Since scale may be variable, it is important to examine to scale to get the proper perspective. Two graphs of the same data with different scales may lead to vastly different ideas, if you are not careful. This concept is often exploited by persons wishing to influence others.

In algebra we usually show that the intersection of the x and y axes is (0,0), the origin. In other situations our data may be nowhere near the origin and it may be difficult to show our graph of data if we include the origin. It is customary in these circumstances to mark the axis, or axes, with a or to draw attention to the break. Sometimes this convention is not followed, and this sometimes leads to errors in interpretation. It is sound practice to mark the axis any time there is a discontinuity.

Compare the following two graphs: The graph on the left does not include the origin and the one on the right does. If you are not careful, you may misread the values of the graph.

Compare the following two graphs: The breaks in the right graph help to show that the origin is not in the image.

Although the importance of a linear scale may be obvious to most, some students each year will present graphs with an axis that is not linear. It is essential that the x and y axes have scales with regular spacing when we make standard scatterplots.

As a final consideration, if you need to plot more than one data set in the same graph, you should vary the symbols so that each set uses a different symbol. You may vary color, use different symbols, such as crosses and dots, or use letters to represent points. Always include a legend so that the reader can easily tell which symbol goes with which set.