In this case, no argument is supplied to geom_point. For example, to create a scatter plot, type: This is sufficient to define a ggplot object, but it does not produce any plotted output until at least one layer is added that specifies a geometry ( geom). A standard call to define the base layer of a ggplot is: More specifically, the ggplot() function has two optional arguments: (i) data (an input data frame), and (ii) aes, which defines the “x” and “y” variables in addition to any other variables to be associated with specific aesthetics, such as color, point shape, or line type. The ggplot() function defines the base layer of a ggplot, indicating the name of the input data frame and establishing the association between a certain subset of its variables and their corresponding roles in the graph. A ggplot object is composed of one or more layers, where each layer contains a different graphical object, or grob for short. Two concepts at the core of ggplot2 are essential for its flexibility and efficiency: layers and aesthetic mappings. A useful companion is the online help pages for ggplot2 ( ), which contains a series of illustrative examples under each help page. Some topics cannot be fully illustrated in the text due to space constraints, so the annotated code will be supplied in the supplemental materials for you to try on your own. The R code and data sets to reproduce all plots shown in this article can be downloaded online (see Supplementary Material online). It helps if the reader is familiar with base and lattice graphics in R, but it is not a prerequisite. R version 2.15.3 and ggplot2 version 0.9.3.1 were used to create all the plots in this article. Click on the link to “ Getting started with qplot.” The chapter in the ggplot2 book 1 corresponding to qplot() is available on the book's web page. For more complex graphics, you should use ggplot(), which is the function used for all of the examples in this article. Its original purpose was to provide a transition from R base graphics to ggplot2 graphics. The former is shorthand for “quick plot” and is particularly useful when you want to create relatively simple graphs. In ggplot2, two functions can be used to create a graphic: qplot() and ggplot(). The creation of a ggplot involves a stepwise process that takes the defined component pieces, called layers, and coordinates them through a sequence of transformations to produce the final graph. Positional adjustments, such as point jittering to reduce overplotting of points or various ways to maneuver bar segments associated with different groups in a bar chart.
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