Lesson 8 of 17 · Week 04 · Thu Oct 15

Graphics with ggplot2

The grammar of graphics

ggplot2 builds a plot from three ingredients: data (a data frame), aesthetic mappings (which columns go to x, y, colour, size…) and geoms (what shape draws each row: points, lines, boxes). You add pieces together with +.

library(ggplot2)
ggplot(data = gm, mapping = aes(x = gdpPercap, y = lifeExp)) +
  geom_point()

In the exercise boxes, click Run and the plot appears under the output. In Positron it appears in the Plots pane.

Adapted from Software Carpentry's “Creating Publication-Quality Graphics with ggplot2” episode (CC BY 4.0, The Carpentries). Text, examples and exercises have been rewritten for this course.

Lines, groups and layers

ggplot(gm, aes(x = year, y = lifeExp, group = country, colour = continent)) +
  geom_line() +
  geom_point()

group = country tells geom_line() to draw one line per country instead of zig-zagging through every point. Layers are drawn in the order you add them: later layers are painted on top of earlier ones.

A mapping inside ggplot() applies to every layer; a mapping inside one geom, e.g. geom_line(aes(colour = continent)), applies to that layer only.

Mapped vs. fixed properties

Inside aes(), a property depends on the data: aes(colour = continent). Outside aes(), it's the same for every point: geom_point(colour = "steelblue", size = 3, alpha = 0.6). Point shapes are numbers (shape = 17 is a filled triangle; see ?points).

Scales and trend lines

ggplot(gm, aes(x = gdpPercap, y = lifeExp)) +
  geom_point(alpha = 0.5) +
  scale_x_log10() +                        # GDP spans 500 to 36,000: use a log scale
  geom_smooth(method = "lm", se = FALSE)   # a straight-line fit (more in Week 05)

If colour = continent is mapped in ggplot(), geom_smooth() fits one line per continent, because it inherits that grouping.

Facets, labels and themes

ggplot(gm, aes(x = year, y = lifeExp, group = country)) +
  geom_line() +
  facet_wrap(~ continent) +                    # one small panel per continent
  labs(x = "Year", y = "Life expectancy (years)", title = "Life expectancy, 1967-2007") +
  theme_minimal() +
  theme(axis.text.x = element_text(angle = 45))

facet_grid(rows ~ cols) lays panels out by two variables. theme(axis.text.x = element_blank()) removes the x-axis labels; axis.title.x = element_blank() removes its title.

Exercises

Each exercise gives you up to 10 checked attempts. Use Run as many times as you like to experiment first, then click Check answer when you're ready. Your exercises use numbers/subsets unique to you, so answers won't match a classmate's exactly.

ggplot(gm, aes(x = gdpPercap, y = lifeExp)) + geom_point()