rnso rnso - 1 year ago 135
R Question

Adding a continuous color gradient legend strip to plot

I am using following data and code to plot a


ddf=structure(list(VAR = structure(1:9, .Label = c("aaa", "bbb",
"ccc", "ddd", "eee", "fff", "ggg", "hhh", "iii"), class = "factor"),
VAL = c(0L, 32L, 64L, 96L, 128L, 160L, 192L, 224L, 256L)), .Names = c("VAR",
"VAL"), class = "data.frame", row.names = c(NA, -9L))


How can I add a legend strip having these colors on it, with corresponding values written there? I want a legend strip as appears in following plot:

dat <- data.frame(x = rnorm(1000), y = rnorm(1000), z = rnorm(1000, mean = 1))
maxz <- max(abs(dat$z))
levelplot(z ~ x * y, dat, at = seq(-maxz, maxz, length = 100), panel = panel.levelplot.points, par.settings = custom.theme.2())

I have tried to use following function examples but am unable to modify them to my needs:

legend("bottom", legend = LETTERS[1:6], col = c6, ncol = 2, cex = 2, lwd = 3, text.font = i, text.col = c6)


Thanks in advance.

Answer Source

The trick is to draw the color key as an image in a separate panel. Notice that I decrese the margin between the panels by reducing par(mar=...) and add an outer margin around both panels with par(oma=...).

my.palette <- rgb(ddf$VAL,256-ddf$VAL,0,maxColorValue=256)

# Create two panels side by side
layout(t(1:2), widths=c(5,1))

# Set margins and turn all axis labels horizontally (with `las=1`)
par(mar=rep(.5, 4), oma=rep(3, 4), las=1)


# Draw the color legend
image(1, ddf$VAL, t(seq_along(ddf$VAL)), col=my.palette, axes=FALSE)

enter image description here

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