Overlay color graphics on 2 color scales ggplot2

Question

Trying to overlay color graphics (something like below) with ggplot2

Trying to use geom_tile, but ggplot does not allow me to add 2 color scales. Here is a sample code.

df <- data.frame(expand.grid(1:5,1:5)) df$z1 <- runif(nrow(df)) df$z2 <- runif(nrow(df)) g1 <- ggplot(df,aes(Var1,Var2)) + theme_bw() #layer 1 g11 <- g1 + geom_tile(aes(fill=z1),alpha=0.5) + scale_fill_gradient(low="white", high="red") #layer 2 g12 <- g1 + geom_tile(aes(fill=z2),alpha=0.5) + scale_fill_gradient(low="white", high="green") g11 g12 

One way to do this is to make 2 layers in different groups. But the result simply does not look intuitive.

 mdf=melt(df,'id'=1:2) g2 <- ggplot(mdf,aes(Var1,Var2,fill = factor(variable),alpha = value)) + geom_tile() + scale_fill_manual(values = c('red','green')) + theme_bw() g2 

Answer 1

You are really close: just don't divide z1 and z2 by 2.

However, there is one more problem. If z1 and z2 are not on the same scale, should you use a common scale for both, or should you scale them independently? The result may be (may be) different, as shown below.

 gg.overlay <- function(df) { # produces 2 color channels and the overlay require(ggplot2) require(gridExtra) gg.z1 <- ggplot(df, aes(x,y))+ geom_tile(fill=rgb(red=df$z1.scale,green=0,blue=0))+ scale_x_continuous(expand=c(0,0))+ scale_y_continuous(expand=c(0,0))+ coord_fixed() gg.z2 <- ggplot(df, aes(x,y))+ geom_tile(fill=rgb(red=0,green=df$z2.scale,blue=0))+ scale_x_continuous(expand=c(0,0))+ scale_y_continuous(expand=c(0,0))+ coord_fixed() gg <- ggplot(df, aes(x,y))+ geom_tile(fill=rgb(red=df$z1.scale,green=df$z2.scale,blue=0))+ scale_x_continuous(expand=c(0,0))+ scale_y_continuous(expand=c(0,0))+ coord_fixed() library(gridExtra) grid.arrange(gg.z1, gg.z2, gg, ncol=3) } 

An example is a bit closer to the image in your question:

 library(mvtnorm) # just for this example df <- expand.grid(x=seq(-3,3,len=100),y=seq(-3,3,len=100)) df$z1 <- with(df,dmvnorm(cbind(x,y),mean=c(0,0),sigma=matrix(c(1,-1,-1,2),nc=2))) df$z2 <- with(df,3*dmvnorm(cbind(x,y),mean=c(0,0),sigma=matrix(c(1,0,0,1),nc=2))) # scale z1 and z2 together max.z <- with(df,max(z1,z2)) min.z <- with(df,min(z1,z2)) df$z1.scale <- with(df, (z1-min.z)/(max.z-min.z)) df$z2.scale <- with(df, (z2-min.z)/(max.z-min.z)) gg.overlay(df) 

 # scale z1 and z2 separately df$z1.scale <- with(df, (z1-min(z1))/diff(range(z1))) df$z2.scale <- with(df, (z2-min(z2))/diff(range(z2))) gg.overlay(df) 

In the first case, the reds are muffled because the intensity z1 lower than the intensity z2 . In the second case, we scale them separately, so the red ones are brighter. It is not clear which one is "correct."

Answer 2

One possible solution is to manually calculate the color and then pass it to geom_tile. (continued to the above code)

 cols=rgb(red=df$z1/2,green=df$z2/2,blue=rep(0,nrow(df))) g1 <- ggplot(df,aes(Var1,Var2)) + theme_bw() + geom_tile(fill=cols) g1 

How brighter are the colors?