Overview

Teaching: 5 min
Exercises: 15 min

Questions

• What kind of scientific question does a faceted table of charts address?

• How do I generate a faceted table with ggplot2?

Objectives

• To generate a faceted table of histograms using ggplot2

Build student independence to walk through decision making steps from question to chart.

• “How was the world population distributed among continents in 2007?”
• Comparison among many items or categories
• Faceted table of histogram bar charts

Implement faceted table of histograms.

• facet_grid()
• lims()

Fix the Faceted Table Code Challenge

Fix the code to generate a faceted table of histograms.

ggplot(pop_2007, aes(year)) +
  geom_histogram(binwidth=0.5) +
  scale_x_log10(limits = c(0.1, 10))
  facet_grid(continent ~ .) +
  labs(x = Population [millions], y = Number of countries, 
       title = "2007") +
  theme_bw(base_size = 42, base_family = "Script") +
  theme(axis.title=element_text(size=36))

Output Solution

Code Solution

ggplot(pop_2007, aes(population/1000000)) +             # Replace aes()
 geom_histogram(binwidth=0.5) +
 scale_x_log10(limits = c(0.1, 1000)) +                 # Add +
 facet_grid(. ~ continent) +                            # Reverse ~ .
 labs(x = "Population [millions]", y = "Number of countries", 
      title = "2007") +                                 # Add ""
 theme_bw(base_size = 24, base_family = "Helvetica") +  # Fix input vars
 theme(axis.title = element_text(size = 36))

Alternate Good Charts Discussion

This is another quality solution to addressing “How was the world population distributed among continents in 2007?”

Compare and contrast the two graphic representations addressing the research question.

Grouped Density Chart Code

ggplot(pop_2007, aes(x = population/1000000, y = ..count.., fill = continent)) +
 geom_density(alpha = 0.6) +
 geom_histogram(data=filter(pop_2007, continent=="Oceania"), alpha = 0.6) +
 scale_x_log10(limits = c(0.1, 1000)) +
 labs(x = "Population [millions]", y = "Number of countries", title = "2007") +
 theme_bw(base_size = 24, base_family = "Helvetica") +
 theme(axis.title = element_text(size = 36))

Key Points

• Faceted tables represent a comparison among many items or categories.

• Multiple good charts can address a single scientific question.

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