Best way to add a footnote to a plot created with ggplot2

There are different kinds of tools for data visualization and ggplot2 is always my favorite. It is powerful, elegant and easy to use except for one minor defect — difficulty of adding footnote. Unlike adding title, it’s no explicit statement available to add a footnote directly. Let’s use the following plot as example (according to the mpg data set included in ggplot2).

toyota <- mpg[which(mpg$manufacturer == 'toyota'), ]
p <- ggplot(toyota, aes(displ, hwy)) + facet_wrap(~ class, ncol = 2) + geom_point(aes(size=cyl))


As you can see, I create a 4-panel scatter plot using displ as x-axis and hwy as y-axis. Let’s see how we can add the footnote to the plot.

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