colors for two geom_point() in ggplot2 when using aes_string

Question

I want to produce a plot from two series 'Pos' and 'Neg' (y values) from a data frame. The x value is in the 'Mean' column. I want the series to have different colour.

Searching stackoverflow gave me a similar question: change color for two geom_point() in ggplot2, but I want to use aes_string in order to avoid notes when checking the package.

I get it to work using aes and 'automatic' colours, as in the first example below. However, I can't figure out how to produce the same plot using aes_string and still let ggplot decide the colours. I feel that this should be a simple thing...

A reproducible example:

exData <- data.frame(Marker = rep("TH01", 10),
                 Mean = seq(1:10),
                 Neg = -1*runif(10,0.1,1),
                 Pos = runif(10,0.1,1))

# Produce the correct plot, with 'automatic' colours.
gp <- ggplot(exData, aes_string(x="Mean"),
             shape=val_shape, alpha=val_alpha)
gp <- gp + geom_point(aes(y=Pos, colour="Max"))
gp <- gp + geom_point(aes(y=Neg, colour="Min"))
gp <- gp + scale_colour_discrete(name = "Legend")
print(gp)

# Produce the correct plot, but not with 'automatic' colours.
gp <- ggplot(exData, aes_string(x="Mean"),
             shape=val_shape, alpha=val_alpha)
gp <- gp + geom_point(aes_string(y="Pos"), colour=1)
gp <- gp + geom_point(aes_string(y="Neg"), colour=2)
gp <- gp + scale_colour_discrete(name = "Legend")
print(gp)

Answer 1

The way your data is formatted is not ideal for ggplot2. Convert it to the "long" format first:

library(reshape2)
exData.m <- melt(exData, id.vars=c("Marker", "Mean"))

ggplot(exData.m, aes(x=Mean, y=value, color=variable)) + geom_point()

As a rule of thumb, each aesthetic (x, y, color, shape, alpha, ...) requires a column in the data frame to be plotted. The reshape2 library is helpful here.