gtable - janeshdev/ggplot2 GitHub Wiki

The R package gtable is designed to help construct and manipulate layouts containing graphical elements. The standard grid package in R provides low-level functions to define viewports, and place graphical elements (grobs) at specific locations within the device window. gtable builds upon these functions and provides a higher-level interface, where one can e.g. merge two layouts, add columns, rows, insert graphical elements in a given cell, and change the display order, among other things.

The gtable package is used internally by ggplot2, and can therefore be used to modify the layout of such plots.

More info can be found on this wiki page