Book Excerpt

Keep your graphs and charts honest by avoiding these common data visualization pitfalls.

Did Reuters and Fox News intend to mislead their audiences by these terrible charts?

One of the earliest known bar graphs, from The Commercial and Political Atlas.

Whether in school or in the workplace, the majority of us have had to either read or create charts and graphs at some point in our lives. This graphic representation of data, called “data visualization” or “data viz,” has been our go-to method for understanding how numbers correlate to one another for centuries. In fact, the first-ever line and bar graphs were developed as early as 1786, when William Playfair released The Commercial and Political Atlas.

Playfair wanted to compare the total amount of exports to the total amount of imports in Scotland over a single-year period (from 1780 to 1781), broken down by source/destination. To chart total numbers, identify locations, and differentiate imports from exports, he realized that a combination of bars and scales would do the trick. The result was the first recorded bar graph in history, pictured above.

At the time, Playfair was navigating uncharted waters, but today, charts and graphs are extremely commonplace. Over the years, tools such as Excel, Tableau, and PowerPoint have made it extremely easy for us to quickly visualize data sets and share them widely. Given this, you might be surprised to learn that mistakes in data visualizations run rampant in the world of visual content. For instance, it’s common to assume that comparing numbers to one another always requires a bar chart, while showing percentages always requires a pie chart. Or sometimes it’s assumed that a bar chart can have multiple scales, or worse, no scale at all.

We have taken charts and graphs for granted. Because they’re so easy to produce, we automatically assume they’re accurate. But tools such as Excel and PowerPoint just visualize the numbers we input; they don’t consider the context of those numbers to determine their best graphical representation.

To identify the right visualization to use, we must first consider the story we are trying to tell, just as Playfair did. We must consider how the associated numbers correlate to one another and to the context in which the…