2.2 Grammar of Graphics

Just as the grammar of language helps us construct meaningful sentences out of words, the Grammar of Graphics helps us to construct graphical figures out of different visual elements. This grammar gives us a way to talk about parts of a plot: all the circles, lines, arrows, and words that are combined into a diagram for visualizing data. Originally developed by Leland Wilkinson, the Grammar of Graphics was adapted by Hadley Wickham to describe the components of a plot, including

• the data being plotted
• the geometric objects (circles, lines, etc.) that appear on the plot
• a set of mappings from variables in the data to the aesthetics (appearance) of the geometric objects
• a statistical transformation used to calculate the data values used in the plot
• a position adjustment for locating each geometric object on the plot
• a scale (e.g., range of values) for each aesthetic mapping used
• a coordinate system used to organize the geometric objects
• the facets or groups of data shown in different plots

Wickham further organizes these components into layers, where each layer has a single geometric object, statistical transformation, and position adjustment. Following this grammar, you can think of each plot as a set of layers of images, where each image’s appearance is based on some aspect of the data set.

All together, this grammar enables us to discuss what plots look like using a standard set of vocabulary. And similar to how tidyr and dplyr provide efficient data transformation and manipulation, ggplot2 provides more efficient ways to create specific visual images.