ggplot_build takes the plot object, and performs all steps necessary
to produce an object that can be rendered. This function outputs two pieces:
a list of data frames (one for each layer), and a panel object, which
contain all information about axis limits, breaks etc.
layer_data(plot, i = 1L)ggplot object
Nicholas Hamilton
layer_data, layer_grob, and layer_scales are helper
functions that returns the data, grob, or scales associated with a given
layer. These are useful for tests.
print.ggplot and benchplot for
functions that contain the complete set of steps for generating
a ggplot2 plot.