3  ggplot2 and the Grammar of Graphics

ggplot2 is built on a grammar of graphics that abstracts data visualizations into separate components.¹

Component	Function	Description
Data	ggplot(data)	The data to be visualized.
Aesthetic mappings	aes()	Aesthetic mappings between variables and visual properties.
Geometries	geom_*()	The geometric shape used to represent the data.
Statistics	stat_*()	Any statistical transformations applied to the data.
Position	position_()	Any positional adjustment: stack, jitter, dodge, etc.
Scales	scale_*()	How aesthetic qualities are mapped to the data.
Guides	guides() & labs()	Labels for axes and legends to help interpret a plot.
Coordinate system	coord_*()	How data coordinates are mapped onto the plane of the graphic.
Facets	facet_*()	Any breaking up of the data into multiple plots.
Theme	theme()/theme_*()	The overall visual defaults of a plot.

Central to how ggplot2 implements the grammar of graphics is the notion of layers, which is used in two different but overlapping ways.

1. As described in the ggplot2 documentation, “A layer combines data, aesthetic mapping, a geom (geometric object), a stat (statistical transformation), and a position adjustment. Typically, you will create layers using a geom_ function, overriding the default position and stat if needed.”
2. You also build plots in ggplot2 by adding layers, using + between the different components of the plot to iteratively develop a visualization.

We can translate the grammar of graphics into generalized ggplot2 code as follows:

ggplot(data = <DATA>,
       mapping = aes(<GLOBAL-MAPPINGS>)) + 
  geom_*(
     mapping = aes(<LOCAL-MAPPINGS>),
     stat = <STAT>, 
     position = <POSITION>
  ) +
  scale_*(<SCALES>, <GUIDES>) + 
  labs(<LABELS>) + 
  coord_*() + 
  facet_*() + 
  theme()

Note that you do not need to specify all of these elements for your plots. ggplot2 will choose reasonable defaults for those elements you do not specify.

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1. Leland Wilkinson, The Grammar of Graphics, Second Edition (Springer-Verlag, 2005), https://doi.org/10.1007/0-387-28695-0; Hadley Wickham, “A Layered Grammar of Graphics,” Journal of Computational and Graphical Statistics 19, no. 1 (2010): 3–28, https://doi.org/10.1198/jcgs.2009.07098; Hadley Wickham, ggplot2: Elegant Graphics for Data Analysis, Second Edition (Springer, 2016), https://doi.org/10.1007/978-3-319-24277-4, page 4.