Grammar of graphics

Jeff Stevens

2023-03-31

Introduction

Set-up

Plotting with {ggplot2}

Grammar of graphics

Seven components/layers of ggplots

  • Data
  • Mappings – maps data to properties of geom
  • Geom – represents data
  • Stat – transforms data
  • Position – control placement of data on coordinate
  • Facet – split graph into subplots
  • Coordinate – places data in coordinate system

Seven components/layers of ggplots

ggplot(data = DATA) +
  GEOM_FUNCTION(
    mapping = aes(MAPPINGS),
    stat = STAT,
    position = POSITION
  ) +
  COORDINATE_FUNCTION +
  FACET_FUNCTION

Full specification of plot

ggplot(data = penguins) +
  geom_point(
    mapping = aes(x = bill_length_mm, y = bill_depth_mm),
    stat = "identity",
    position = "identity"
  ) +
  coord_cartesian() +
  facet_null()

Data

Tidy data

Data should be in tidy format for ggplots

mpg
# A tibble: 234 × 11
   manufacturer model      displ  year   cyl trans drv     cty   hwy fl    class
   <chr>        <chr>      <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr>
 1 audi         a4           1.8  1999     4 auto… f        18    29 p     comp…
 2 audi         a4           1.8  1999     4 manu… f        21    29 p     comp…
 3 audi         a4           2    2008     4 manu… f        20    31 p     comp…
 4 audi         a4           2    2008     4 auto… f        21    30 p     comp…
 5 audi         a4           2.8  1999     6 auto… f        16    26 p     comp…
 6 audi         a4           2.8  1999     6 manu… f        18    26 p     comp…
 7 audi         a4           3.1  2008     6 auto… f        18    27 p     comp…
 8 audi         a4 quattro   1.8  1999     4 manu… 4        18    26 p     comp…
 9 audi         a4 quattro   1.8  1999     4 auto… 4        16    25 p     comp…
10 audi         a4 quattro   2    2008     4 manu… 4        20    28 p     comp…
# ℹ 224 more rows

Data to ggplot

Data inside ggplot()

ggplot(data = mpg)

Data to ggplot

Data piped to ggplot()

mpg |>
  ggplot()

Data to ggplot

Process data before plotting

mpg |>
  filter(class != "2seater") |>
  mutate(class = str_to_sentence(class)) |>
  ggplot()

Mapping

Map data to positions

Specify columns for x and y

#
ggplot(data = mpg, mapping = aes(x = displ, y = hwy))

Map data to positions

Equivalent but not ideal. Why?

#
ggplot(mpg, aes(displ, hwy))

Map data to positions

This is how we’ll do it

mpg |>
  ggplot(aes(x = displ, y = hwy))

Geometric objects

Geoms

There are many different ways of representing data on a plot

Plot points

Add geom_point()

mpg |>
  ggplot(aes(x = displ, y = hwy)) +
  geom_point()

Plot points

How is this different? What are advantages/disadvantages?

mpg |>
  ggplot() +
  geom_point(aes(x = displ, y = hwy))

Plot smooth lines

#
mpg |>
  ggplot(aes(x = displ, y = hwy)) +
  geom_smooth()

Plot multiple geoms

mpg |>
  ggplot(aes(x = displ, y = hwy)) +
  geom_point() +
  geom_smooth()

Order matters

mpg |>
  ggplot(aes(x = displ, y = hwy)) +
  geom_smooth() +
  geom_point()

Plot linear regression line

mpg |>
  ggplot(aes(x = displ, y = hwy)) +
  geom_point() +
  geom_smooth(method = "lm")

Plot boxplots

mpg |>
  ggplot(aes(x = class, y = displ)) +
  geom_boxplot()

Let’s code!

Grammar of graphics [Rmd]

DPaViR 2023