Look at Data

Andrew Irwin, a.irwin@dal.ca

2026-01-15

Principles of data visualization

  • What can you learn more easily from a visualization than from a table of data?

  • What makes a visualization good?

  • What features of human visual perception should be considered when designing data visualizations?

Why visualize data?

Anscombe’s Quartet

set n x_mean x_sd y_mean y_sd correlation
1 11 9 3.3166 7.5009 2.0316 0.8164
2 11 9 3.3166 7.5009 2.0317 0.8162
3 11 9 3.3166 7.5000 2.0304 0.8163
4 11 9 3.3166 7.5009 2.0306 0.8165

Anscombe’s Quartet

Datasaurus Dozen

Figure 1: The datasaurus dozen, all with the same mean, standard deviation, and correlation.

Datasaurus Dozen

Figure 2: The secret revealed

What can you learn from a visualization?

  • Relationships between variables
  • Relative values of means, standard deviation, …
  • Trends and patterns
  • … your ideas here

What is a good visualization?

Still from CBC Canadian Federal election coverage, October 21, 2019. Does the three-dimensional bar chart help you understand the election outcome? Are the bars the right height? Is it helpful to have the bar chart appear in the middle of the studio set? Can you judge the height of the bars?

Visual perception

Healy. Fig. 1.24.

Proportions: bar and pie charts

Figure 3: Wilke. Fig. 10.4.

Proportions: bar and pie charts

Figure 4: Wilke. Fig. 10.6.

Figure 5: The checker shadow illusion.

Figure 6: The checker shadow illusion.

Figure 7: The checker shadow illusion.

Bars or dots?

Healy. Fig. 1.26.

Aspect ratio

Healy. Fig. 1.12.

Where to put 0?

Figure 8: Healy. Fig. 1.27.

Where to put 0?

Figure 9: Healy. Fig. 1.27.

How to make a good visualization

  • Think about your goals.
  • What are the best choices to highlight the most important features of the dataset?
  • How will your reader interpret your visualization?
  • Does your picture represent the data fairly?
  • Are there perceptual problems that make the picture easy to misinterpret or difficult to understand?
  • Revise and edit your graphs.
  • Practice thinking about the effectiveness of visual elements.

Suggested reading

  • Course notes: Look at Data

  • Healy. Chapter 1: Look at Data (Strongly recommended)

  • Wilke. Chapter 1: Introduction

  • Wilke. Chapters 5-16: “Visualizing” series of chapters. Return to these over time.

Exercises

  • Practice using ideas about the kind of visualization, position, shape, colour, order to assess what messages are most readily communicated by a visualization

  • Practice thinking about how alternative displays of the same data will change the message of a visualization

  • How easy is it for you, or less experienced users of visualizations, to understand a visualization?