par(mfrow=c(r,c)) for base graphics, where
r is the number of rows and
c the number of columns.
Other resources including learning about
layouts. Multipanel plotting with base graphics http://seananderson.ca/courses/11-multipanel/multipanel.pdf
grid.arrange function in the
gridExtra package. I’ve done it several times above. You assign the output of a ggplot object to an object (here it’s
plot2). Then you use
grid.arrange() to arrange them either side by side or top and bottom.