A. Grammar or graphics

When creating a plot we start with data. Given four variables A,B,C and D, the first step in making this plot is to create a new dataset that reflects the mapping of x-position to A, y-position to C, and shape to D. x-position, y-position, and shape are examples of aesthetics, things that we can perceive on the graphic.

The next thing we need to do is to convert these numbers measured in data units to numbers measured in physical units, things that the computer can display. To do that we need to know that we are going to use linear scales and a Cartesian coordinate system. We can then convert the data units to aesthetic units, which have meaning to the underlying drawing system. These transformations are the responsibility of scales.

Finally, we need to render these data to create the graphical objects that are displayed on screen or paper. To create a complete plot we need to combine graphical objects from three sources: the data, represented by the point geom; the scales and coordinate system, which generates axes and legends so that we can read values from the graph; and the plot annotations, such as the background and plot title. Combining and displaying these graphical objects produces the final plot.

Faceting is a more general case of the techniques known as conditioning, trellising, and latticing, and produces small multiples showing different subsets of the data. If we facet the previous plot by D we will get a plot where each value of D is displayed in a different panel.

Components of the layered grammar

The layered grammar of ggplot2 defines the components of a plot as:

  • a default dataset and set of mappings from variables to aesthetics,
  • one or more layers, with each layer having one geometric object, one statistical transformation, one position adjustment, and optionally, one dataset and set of aesthetic mappings,
  • one scale for each aesthetic mapping used,
  • a coordinate system,
  • the facet specification.

Layers are responsible for creating the objects that we perceive on the plot. A layer is composed of four parts:

  • data and aesthetic mapping,
  • a statistical transformation (stat),
  • a geometric object (geom), and
  • a position adjustment.

B. ggplot()

There are three common ways to invoke ggplot:

  • ggplot(df, aes(x, y, ))
    • if all layers use the same data and the same set of aesthetics
  • ggplot(df)
    • when one data frame is used predominantly as layers are added, but the aesthetics may vary from one layer to another.
  • ggplot()
    • when multiple data frames are used to produce different layers

Usage

ggplot() +
  layer(
      data = diamonds, mapping = aes(x = carat, y = price),
      geom = "point", stat = "identity", position = "identity"
  ) +
  layer(
    data = diamonds, mapping = aes(x = carat, y = price),
    geom = "smooth", position = "identity",
    stat = "smooth", method = lm
  ) +
  scale_y_log10() +
  scale_x_log10() +
  coord_cartesian()

Shorthanded:

ggplot(diamonds, aes(carat,price)) +
  geom_point() +
  stat_smooth(method=lm) +
  scale_x_log(10) +
  scale_y_log(10)

C. qplot()

Usage

qplot(x, y = NULL, ..., data, facets = NULL, margins = FALSE,
  geom = "auto", stat = list(NULL), position = list(NULL), 
  xlim = c(NA,NA), ylim = c(NA, NA), log = "", main = NULL,
  xlab = deparse(substitute(x)), ylab = deparse(substitute(y)), asp = NA)

Arguments

[Arguments for qplot()]

arg description
x x values
y y values
… other aesthetics passed for each layer
data data frame to use (optional). If not specified, will create one, extracting vectors from the current environment.
facets faceting formula to use. Picks facet_wrap or facet_grid depending on whether the formula is one sided or two-sided
margins whether or not margins will be displayed
geom character vector specifying geom to use. Defaults to “point” if x and y are specified, and “histogram” if only x is specified.
stat character vector specifying statistics to use
position character vector giving position adjustment to use
xlim limits for x axis
ylim limits for y axis
log which variables to log transform (“x”, “y”, or “xy”)
main character vector or expression for plot title
xlab character vector or expression for x axis label
ylab character vector or expression for y axis label
asp the y/x aspect ratio

Sources

Hadley WICKHAM. A Layered Grammar of Graphics