Using this script
df <- data.frame(x = 1:5, y = 1:5, color = letters[1:5])
ggplot(df, aes(x, y, fill = color)) +
geom_point(size = 4, pch = 21) +
gui
quick and ugly answer but it does the job until someone post better...
ggplot(df, aes(x, y, fill = color)) +
geom_point(size = 4, pch = 21) +
guides(fill = guide_legend(
title = expression(atop(Median~Nitrate-Nitrogen~(NO[3]^{textstyle("-")}-N), "Concentration"~(mg~L^{textstyle("-")})~phantom (1000000)~phantom (1000000)))))