—Data is the most important thing, and the thing that you bring to the table.
—Geometric objects … what you actually see on the plot: points, lines, polygons, etc.
—Statistics transform the data in many useful ways. For example, binning and counting to create a histogram….
—Scales map values in the data space to values in an aesthetic space, whether it be colour, or size, or shape. Scales also provide an inverse mapping: a legend.
—A coordinate system describes how data coordinates are mapped to the plane of the graphic. It also provides axes and gridlines to make it possible to read the graph.
— A facetting, or conditioning, specification…. to reproduce the same plot for different subsets of the data. The facetting specification describes those subsets and how the facets should be arranged in to a plot.