Present Beyond Measure

24Avoid the Most Common Visualization Violations

“Rules are for fools to follow and for wise men to be guided by.”

—Winston Churchill

You've already learned how to avoid several data viz no-nos in the PICA Protocol; however, there are still a few more to consider. These chart types and practices are considered violations of how the brain most effectively receives information, and they are more common than you think.

I'm going to ease you into the violation section with perhaps the one you'll feel the most uncomfortable with. Why? To rip off the Band-Aid! Once you move through this one, it's relatively painless from here.

Viz Violation: Dual-Axis Charts with Different Scales

The dual-axis chart consists of two different chart types overlaid in the same plot space using two different scales (usually a bar and a line). This is known as a combo chart in Excel and PowerPoint. Figure 24.1 shows an example.

FIGURE 24.1 Example of dual-axis bar and line graph with different scales

I'm quite familiar with the appeal of this viz; in my agency days, I used dual-axis bar/line charts like it was my job. Because … it was my job. Stakeholders wanted more, more, more, and these charts enabled me to squeeze as much data as possible into an itty-bitty living space, especially on dashboards. I'll show you why that's really a drawback, not a benefit.

Our eyes and brains are wired to draw ...