A Grammar of Graphics

STAT 20: Introduction to Probability and Statistics

Agenda

• Announcements
• Reading Questions
• Break
• Appendix: More practice!
• Worksheet

Anouncements

• Portfolio 2 due Friday

• Lab 2 due Next Tuesday (I had originally said this Friday, but we will cover things in tomorrow’s lecture that are needed so a bit longer due date)

• Quiz 1 this week on Thursday
• Practice problems released tomorrow

Concept Questions

Which of these geometries typically go with aesthetic mappings where there are two numerical variables, one on the x axis and the other on the y axis? Select all that apply.

• A. Violin
• B. Dot
• C. Point
• D. Line
• E. Box

00:45

How is a stacked, normalized bar chart defined in terms of the grammar of graphics?

• A. one categorical variable mapped to the x-axis, using the bar geometry.

• B. one categorical variable mapped to the x-axis, a second categorical variable mapped to the y-axis, using the bar geometry.

• C. one categorical variable mapped to the x-axis, a second categorical variable mapped to the fill, using the bar geometry.

• D. numerical variable mapped to the x-axis, a second categorical variable mapped to the fill, using the histogram geometry.

00:30

Which of the following describes or is an example of mapping?

• A. Adding a theme to a plot.

• B. The linking of the aesthetic attributes of a plot to the data in a column of a data frame.

• C. Adding an annotation to a plot.

• D. The tweaking of all points on a scatter plot in a cosmetic manner.

00:30

True or False: A visualization using the grammar of graphics may contain more than one aesthetic mapping, but not more than one geometry.

• A. True
• B. False

00:30

When conducting exploratory data analysis, who is generally the audience for your plots?

• A. Other scientists

• B. Yourself

• C. The public

• D. Your boss

• E. Oski

00:30

Break

05:00

Appendix - more practice!

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

01:00

ggplot(penguins, aes(x = bill_length_mm, y = species)) + geom_boxplot()

Correct answer: Bill length is mapped to the x, species is mapped to the y, and it uses the boxplot geometry.

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?

01:00

Correct answer: ggplot(penguins, aes(x = island, fill = island)) + geom_bar()

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:

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.

If you don’t have much time, begin the video at 2:19 and stop at 4:53. If you have a bit of time, begin the video at the beginning, but still, stop at 4:53.

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

01:00

Change timer to at least 5:00. Then go on to the next slide, but pause with the slide still blank. Ask them what were the variables and aesthetic attributes, and write them down on the board.

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

Worksheet: A Grammar of Graphics

25:00