A Grammar of Graphics

STAT 20: Introduction to Probability and Statistics

Concept Questions

1. What are the aesthetics and geometry of this plot?

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Correct answer: see code above.

This is a warm up to get them thinking about the fundamental concept of aesthetic mappings and geometries on a plot style and data set they’ll be familiar with. This should be quite easy.

What code was used to make this plot?

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Correct answer: see above.

This demonstrates a practice called “double-mapping”, when the same column is mapped to two different aesthetic attributes. Most consider this a bad practice.

In a setting like this, usually the author is really just trying to get more color onto the plot. That can be accomplished by setting the color of the bars using something like:

Global infectious disease prevalence in 1989

2. What are the aesthetics and geometry of this plot?

01:00

Correct answer: Color is mapped to taxonomic family, size is mapped to prevalence, the diseases are represented using the point geometry.

Good follow up questions and boardwork include:

• What does the data frame look like that made this plot?
• What determines where the bubbles are placed in the chart? Answer: bubble charts have different methods of doing this. This one appears to use a packing algorithm so get them close together. Important, none of those methods refer to a column in the data frame therefore are not aesthetic mappings.
• If you really want to go far afield, you could show an example of a beeswarm chart, which is a bubble chart that uses one of the x or the y to map a numerical variable and then uses a packing algorithm to determine placement on the other axis.
• One of the lessons of this bubble chart is that our “named” charts correspond to particular combos of aesthetic mappings and geometriess. A “scatterplot” uses the point geometry and maps numerical data to the x and y axes. A “bubble chart” uses the point geometry too, but maps size to a numerical variable (and often uses color too).

Concept Activity

Concept Activity

You will be watching a 2.5 minute video of a presentation by a scientist, Hans Rosling, who studied global public health. He presents data visualizations depicting the change in life expectancy and family size over several decades in the 20th century.

On a piece of note paper:

• Sketch out the data frame used to create the graphic and add the names of the variables.
• List the aesthetic attributes used to encode the data in the graphic.
• Identify the geometry used in the plot.

Please turn to your neighbors and…

Discuss what you came up with in terms of . . .

• the variables present in the data frame
• the aesthetic attributes used to encode that data in the plot
• the geometry

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What were the variables and aesthetic attributes?

Visual Cues / Aesthetics

• Location along the x-axis
• Location along the y-axis
• Size of point
• Color of point
• Animation

Variables

• Fertility rate
• Life expectancy
• Population
• Region
• Year

What did the data frame look like?

What was the unit of observation? What were the variables? What were their type?

Unit of observation

• A country in a given year

Variables

• Fertility rate (continuous)
• Life expectancy (continuous)
• Population (continuous)
• Region (nominal)
• Year (discrete)

What geometry is used to represent the observations?

• Points

Quiz Recap

• Head to pollev.com for a series of former quiz questions and quiz-level questions! Make sure you take notes.

What type of claim was made?

The group Morning Consult conducted a poll of a representative sample of registered 1,256 Republican voters on August 24, 2023, asking them who they planned to vote for in the primary eleection. 58% of respondents replied “Trump” and 14% replied “DeSantis”. A major news outlet ran a headline, “Trump leads DeSantis by 44 points among registered Republican voters.” . . .

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What type of variable is listeners?

{fig-alt=“A dataframe with a column labelled ‘Listeners (in million)’ with values 40, 66, 60, 73, 57, 75, and 13’}

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What type of proportion is used?

Roughly 68 percent of those passengers who were in the first class survived the wreckage of the Titanic.

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Which measure of center/spread is least appropriate?

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What are the aesthetics and geometry of this plot?

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What has not changed when moving from left to right?

Break

05:00

Worksheet: A Grammar of Graphics

15:00