For plotting a color bar by itself. Adapted from http://www.colbyimaging.com/wiki/statistics/color-bars.

colorbar.R

colorbar <- function(pal, min, max = -min, nticks = 11, at = seq(min, max, len = nticks), title = '', border = NA, dir = 'v') {
	scale = (length(pal)-1)/(max-min)
	graphics.off()
	if (dir == 'v') {
		w <- 1; h <- 5
		xx <- c(0, 10)
		yy <- c(min, max)
		asp <- 2
		side <- 4
	} else if(dir == 'h') {
		w <- 5; h <- 1
		xx <- c(min, max)
		yy <- c(0, 10)
		asp <- 0.5
		side <- 1
	} else {
		stop("dir must either be 'v' (vertical) or 'h' (horizontal).")
	}
	dev.new(width = w, height = h)
	plot(xx, yy, type = 'n', bty = 'n', xaxt = 'n', xlab = '', yaxt = 'n', ylab = '', main = title, asp = asp)
	axis(side = side, at = at, las = 1, pos = 5, col = NA, col.ticks = NA)
	for (i in 1:(length(pal)-1)) {
		if (dir == 'v') { y = (i-1)/scale + min; rect(0, y, 10, y+1/scale, col = pal[i], border = NA) }
		if (dir == 'h') { x = (i-1)/scale + min; rect(x, 0, x+1/scale, 10, col = pal[i], border = NA) }
	}
	if (dir == 'v') rect(0, 0, 10, y+1/scale, col = NA, border = border)
	if (dir == 'h') rect(0, 0, x+1/scale, 10, col = NA, border = border)
}

Examples

To plot a horizontal color bar, ranging from 0 to 100, with the viridis color palette:

colorbar(pal = viridis::inferno(256), min = 0, max = 100, nticks = 6, dir = 'h')

To add a black border:

colorbar(pal = viridis::inferno(256), min = 0, max = 100, nticks = 6, border = 'black', dir = 'h')

To plot the color bar vertically:

colorbar(pal = viridis::inferno(256), min = 0, max = 100, nticks = 6, border = 'black', dir = 'v')