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How can I use grid to edit a ggplot2 object to add math expressions to facet labels?
levels(parameters) <- c(expression(alpha), expression(beta))
p + facet_grid(.~parameters, labeller = label_parsed)

p + facet_wrap(.~parameters, labeller = label_parsed)  ,  or
p + facet_wrap(.~parameters)

library("ggplot2")

d <- ggplot(iris, aes(Sepal.Length, Sepal.Width)) +
            geom_point() +
            facet_wrap(~Species)
grob <- ggplotGrob(d)
strip_elem <- grid.ls(getGrob(grob, "strip.text.x", grep=TRUE, global=TRUE))$name

grob <- geditGrob(grob, strip_elem[1], label=expression(alpha[1]))
grob <- geditGrob(grob, strip_elem[2], label=expression(beta^2))
grob <- geditGrob(grob, strip_elem[3], label=expression(hat(gamma)))

grid.draw(grob)