How do I generate a mapping from numbers to colors in R?

Question

In R, I can easily generate a color ramp, for example colorRampPalette. The following produces a sequence of five colors, from blue to red:

> mypal <- colo         


        

Answer 1

Here's a map2color function:

map2color<-function(x,pal,limits=NULL){
    if(is.null(limits)) limits=range(x)
    pal[findInterval(x,seq(limits[1],limits[2],length.out=length(pal)+1), all.inside=TRUE)]
}


 map2color(0:11,rainbow(200),limits=c(1,10))
[1] "#FF0000FF" "#FF0000FF" "#FFA800FF" "#ADFF00FF" "#05FF00FF" "#00FFA3FF"
[7] "#00ABFFFF" "#0003FFFF" "#A600FFFF" "#FF00B0FF" "#FF0008FF" "#FF0008FF"
 map2color(0:11,rainbow(200))
[1] "#FF0000FF" "#FF8A00FF" "#EBFF00FF" "#61FF00FF" "#00FF29FF" "#00FFB3FF"
[7] "#00BAFFFF" "#0030FFFF" "#5900FFFF" "#E300FFFF" "#FF0091FF" "#FF0008FF"


mypal <- colorRampPalette( c( "blue", "red" ) )( 5 )
x <- c( 1, 9, 8.5, 3, 3.4, 6.2 )
map2color(x,mypal)
"#0000FF" "#FF0000" "#FF0000" "#3F00BF" "#3F00BF" "#BF003F"

I needed something like this to create a colorramp that would be consistent across different samples of data, so I added an xlim parameter. Thanks for the reminder of the findInterval function in your question.

Answer 2

The simplest thing I can think of is discretizing your variable x with cut and then using the resultant factor to index a color Palettes like heat.colors or cm.colors

f <- function(x,n=10){
    heat.colors(n)[cut(x,n)]
}

see ?heat.colors for more details of Palettes.

This function will give you are color vector corresponding to x of varying granularity proportional to n.

Answer 3

My colourscheme package on R-Forge does that:

https://r-forge.r-project.org/projects/colourscheme/

its primary purpose is to create functions that map numeric values (not just integer colour numbers) to colours. The vignette is the best documentation once you've got it installed.

Answer 4

You can try

> colorRampPalette(c('blue', 'red'))(length(x))[rank(x)]
[1] "#0000FF" "#FF0000" "#CC0032" "#3300CC" "#660099" "#990065"