Colorbar from custom colorRampPalette

Question

I have defined a colorRampPalette:

my.colors = colorRampPalette(c("light green", "yellow", "orange", "red"))

How can I plot a colorbar "legend" item for it, preferably using only the base packages? I am after a rectangle filled with that color gradient.

What I am really after is a way to produce the same type of legend (color bar) that is plotted with a "raster" raster:

require(raster)
plot(raster("myfile.tif"), legend=T)

I need to be able to place this on top of another plot.

It's funny how difficult this is to do in base R. Usually R has so much magic but apparently not for colorbars. I guess statisticians only think in black and white? haha — wordsforthewise, Jan 18 '18 at 23:15

score 35 · Accepted Answer · answered Feb 16 '12 at 16:17

35

I made a nice flexible function awhile ago to do this.

# Function to plot color bar
color.bar <- function(lut, min, max=-min, nticks=11, ticks=seq(min, max, len=nticks), title='') {
    scale = (length(lut)-1)/(max-min)

    dev.new(width=1.75, height=5)
    plot(c(0,10), c(min,max), type='n', bty='n', xaxt='n', xlab='', yaxt='n', ylab='', main=title)
    axis(2, ticks, las=1)
    for (i in 1:(length(lut)-1)) {
     y = (i-1)/scale + min
     rect(0,y,10,y+1/scale, col=lut[i], border=NA)
    }
}

Then you can do something like:

> color.bar(colorRampPalette(c("light green", "yellow", "orange", "red"))(100), -1)

enter image description here

More examples at: http://www.colbyimaging.com/wiki/statistics/color-bars

answered Feb 16 '12 at 16:17

John Colby

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1

Your axis is nicer than mine. +1. – Spacedman Feb 16 '12 at 16:33
4

If you take out the dev.new you can use subplot from package:Hmisc to overlay this onto existing plots. – Spacedman Feb 16 '12 at 18:05
@Spacedman how can you do that? I'm always encountering this error `Error in plot.new() : plot region too large` – gabt Aug 08 '19 at 14:17
It's kind of unclear to me what "lut" means. The link no longer works. – John Polo Nov 17 '20 at 01:45

score 10 · Answer 2 · answered Feb 16 '12 at 16:13

Make a matrix, use image with some axes parameters...

my.colors = colorRampPalette(c("light green", "yellow", "orange", "red"))
z=matrix(1:100,nrow=1)
x=1
y=seq(3,2345,len=100) # supposing 3 and 2345 are the range of your data
image(x,y,z,col=my.colors(100),axes=FALSE,xlab="",ylab="")
axis(2)

image legend

Brad Horn · Answer 3 · 2014-07-23T04:35:09.217

colorbar.plot() from the fields package is a standard solution. Many folks will be looking here for solutions that place the legend outside of the standard plot area or the original x/y range. This limitation of colorbar.plot() is easily overcome by:

plotting your image(),
expanding the size of the plot device with par(pin = c(width, length))
calling colorbar.plot() using original x and y values

Because the area of the plot device is now expanded, the legend will appear outside of the original image() plot. Experimenting with x and y values lands the legend anywhere you wish. The gradient legend can be horizontal or vertical using standard argument options.

Yes but you also need the 'strip' argument and so far it's not as simple as it should be to figure out. A working code example is usually part of a good answer. — wordsforthewise, Jan 18 '18 at 23:12

score 0 · Answer 4 · answered Dec 14 '14 at 18:16

I know this is a very old question, and the answers are all great. I just wanted to point out that you could just smooth the color bar from levelplot.

Make some fake data and a color gradient

mat = matrix(rnorm(400,1,1), ncol=20)
grad = rev(rainbow(1000, start=rgb2hsv(col2rgb('green'))[1], 
       end=rgb2hsv(col2rgb('blue'))[1]))

Make the plot and specify breaks in the color bar

levelplot(mat, col.regions=grad, colorkey = list(at=seq(0,max(mat),length.out=400)))

This functionality to levelplot may have been added long after all you folks came up with sweet workaround solutions.

Colorbar from custom colorRampPalette

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