If print.four.plots.per.page is set to FALSE, the function returns a list of ggplot objects, one element per plot. That allows you to index any individual plot or plots, then modify them as you wish. When print.four.plots.per.page is set to FALSE (the default), the function prints four plots per page on a graphical device but returns NULL.
By default, print.four.plots.per.page is set to TRUE.
[Currently], it reads:
By default,
print.four.plots.per.page
is set toTRUE
.