ggplot2 Cheat Sheet:
Scales

library(ggplot2)

Manual Fill

p <- ggplot(mpg, aes(fl))
p <- p + geom_bar(aes(fill = fl))
p <- p + scale_fill_manual(
    values = c("skyblue", "royalblue", "blue", "navy"),
    limits = c("d", "e", "p", "r"),
    breaks = c("d", "e", "p", "r"),
    name = "fuel",
    labels = c("D", "E", "P", "R")
)
print(p)

Continuous Fill

p <- ggplot(faithfuld, aes(waiting, eruptions, fill = density))
p <- p + geom_tile()
p <- p + scale_fill_continuous(low = "blue", high = "red")
print(p)

Specified Colours

df <- data.frame(
    "x" = 1:4,
    "y" = 1:4,
    "col" = c("red", "green", "blue", "black")
)
p <- ggplot(df, aes(x, y, colour = col))
p <- p + geom_point()
p <- p + scale_colour_identity()
print(p)

Log 10 Axis

df <- data.frame(
    "x" = 1:10,
    "y" = exp(1:10)
)
p <- ggplot(df, aes(x, y))
p <- p + geom_point()
p <- p + scale_y_log10()
print(p)

Square Root Axis

df <- data.frame(
    "x" = 1:10,
    "y" = (1:10)^2
)
p <- ggplot(df, aes(x, y))
p <- p + geom_point()
p <- p + scale_y_sqrt()
print(p)

Reverse Axis

p <- ggplot(mpg, aes(cty, hwy))
p <- p + geom_point()
p <- p + scale_x_reverse()
print(p)

Custom Symbols

p <- ggplot(mpg, aes(cty, hwy))
p <- p + geom_point(aes(shape = fl, size = cyl))
p <- p + scale_shape() + scale_size()
p <- p + scale_shape_manual(values = c(0:25))
print(p)

Custom Size

p <- ggplot(mpg, aes(cty, hwy))
p <- p + geom_point(aes(shape = fl, size = cyl))
p <- p + scale_radius() + scale_size_area()
print(p)