multiple colors in axes titles in ggplot

Question

How can I have multiple colors for an axes label in ggplot?

As an example, instead of a legend, I would like the y-axis label to be red and green to correspond to th

Answer 1

Extending David Arenburg's comment, this gets a little closer without getting into the grid level:

library(reshape2)  # easier to group by color with melted data

temp <- melt(mpg[mpg$model=="a4", c("trans", "cty", "hwy")], id.vars = "trans")

making label strings identified by the same variable for grouping:

labs <- data.frame(variable = c("cty", "hwy"), 
                   value = c("MPG (city); ", "MPG (hwy) "), 
                   y = c(22,26))  # vertical position for labels



p <- ggplot(temp, aes(x=trans, y=value, color=variable)) +
  geom_point() + 
  geom_text(data = labs, angle = 90, # add rotated text near y-axis
            aes(x = 0.5, y = y, label = value)) +
  scale_color_manual(values=c("red", "dark green"), guide = "none") +
  ylab("") # hide default y-axis label

Who says y-axis can't be labeled on the right side, anyhow?