How can I use different color palettes for different layers in ggplot2?

Question

Is it possible to plot two sets of data on the same plot, but use different color palettes for each set?

testdf <- data.frame( x = rnorm(100), 
                  y1 = rnorm(100, mean = 0, sd = 1), 
                  y2 = rnorm(100, mean = 10, sd = 1),
                  yc = rnorm(100, mean = 0, sd = 3))
ggplot(testdf, aes(x, y1, colour = yc)) + geom_point() +
  geom_point(aes(y = y2))

What I would like to see is one set of data, say y1, in blues (color set by yc), and the other set in reds (again color set by yc).

The legend should then show 2 color scales, one in blue, the other red.

Thanks for your suggestions.

Answer 1

If you translate the "blues" and "reds" to varying transparency, then it is not against ggplot's philosophy. So, using Thierry's Moltenversion of the data set:

ggplot(Molten, aes(x, value, colour = variable, alpha = yc)) + geom_point()

Should do the trick.

Answer 2

That's not possible with ggplot2. I think it against the philosophy of ggplot2 because it complicates the interpreatation of the plot.

Another option is to use different shapes to separate the points.

testdf <- data.frame( x = rnorm(100), 
                      y1 = rnorm(100, mean = 0, sd = 1), 
                      y2 = rnorm(100, mean = 10, sd = 1),
                      yc = rnorm(100, mean = 0, sd = 3))
Molten <- melt(testdf, id.vars = c("x", "yc"))
ggplot(Molten, aes(x, value, colour = yc, shape = variable)) + geom_point()