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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