Which function is used to add captions to a plot built with the ggplot2 package in R?

The facet_grid() function is utilized for creating a grid of plots based on the values of one or more categorical variables. It allows for the visualization of subsets of data across multiple panels but does not provide functionality for adding captions or other text annotations to the plot itself.

The geom_line() function is responsible for adding line geometries to a ggplot2 plot, primarily used for creating line graphs. While it contributes to the visual representation of data trends, it does not facilitate the addition of captions or textual elements needed for comprehensive plot descriptions.

The aes() function defines aesthetic mappings for a ggplot2 plot, such as how variables are represented visually (e.g., color, size, shape). While it is a critical component of the ggplot2 framework, it does not handle the addition of captions or any other text-based annotations to enhance the plot's information.