ggplot2.Rdggplot() initializes a ggplot object.
aes() Aesthetic mappings describe how
variables in the data are mapped to visual properties (aesthetics) of
geoms.
geom_point() The point geom is used to
create scatterplots.
geom_area() For each x value,
geom_area() displays a y interval defined by 0 and ymax (or 0 and xmax).
geom_smooth() For detecting patterns in
data.
geom_boxplot() The boxplot compactly
displays the distribution of a continuous variable.
facet_grid() forms a matrix of panels
defined by row and column faceting variables.
facet_wrap() wraps a 1d sequence of
panels into 2d.
vars() Just like aes(), vars() is a quoting
function that takes inputs to be evaluated in the context of a dataset.
To learn more about ggplot2 https://r4ds.had.co.nz/index.html and http://www.cookbook-r.com/Graphs/