Area Plots with Intensity Coloring

July 13, 2010
By

(This article was first published on CYBAEA Data and Analysis, and kindly contributed to R-bloggers)

I am not sure apeescape’s ggplot2 area plot with intensity colouring is really the best way of presenting the information, but it had me intrigued enough to replicate it using base R graphics.

The key technique is to draw a gradient line which R does not support natively so we have to roll our own code for that. Unfortunately, lines(..., type="l") does not recycle the colour col= argument, so we end up with rather more loops than I thought would be necessary.

(The answer is not to use lines(..., type="h") which, confusingly, does recycle the colour col= argument. This one had me for a while, but the type=h lines always start from zero so you do not get the gradient feature.)

We also get a nice opportunity to use the under-appreciated read.fwf function.

##!/usr/bin/Rscript
## nino.R - another version of http://bit.ly/9P9Gh1
## Copyright © 2010 Allan Engelhardt (http://www.cybaea.net/)

## Get the data from the NOAA server
nino <- read.fwf("http://www.cpc.noaa.gov/data/indices/wksst.for",
                 widths=c(-1, 9, rep(c(-5, 4, 4), 4)),
                 skip=4,
                 col.names=c("Week",
                     paste(rep(c("Nino12","Nino3","Nino34","Nino4"), rep(2, 4)),
                           c("SST", "SSTA"), sep=".")))

## Make the date column something useful
nino$Week <- as.Date(nino$Week, format="%d%b%Y")

## Make colour gradients
ncol <- 50
grad.neg <- hsv(4/6, seq(0, 1, length.out=ncol), 1) # Blue gradient
grad.pos <- hsv(  0, seq(0, 1, length.out=ncol), 1) # Red gradient

## Make plot
plot(Nino34.SSTA ~ Week, data=nino, type="n",
     main="Nino34", xlab="Date", ylab="SSTA", axes=FALSE)
do.call(function (...) rect(..., col="gray85", border=NA),
        as.list(par("usr")[c(1, 3, 2, 4)]))

y <- nino$Nino34.SSTA                   # The values we will plot
x <- nino$Week

axis.Date(1, x=x, tck=1, col="white")
axis(2, tck=1, col="white")
box()

idx <- integer(NROW(nino))
idx[y >= 0] <- 1 + round( y[y >= 0] * (ncol - 1) / max( y[y >= 0]), 0)
idx[y <  0] <- 1 + round(-y[y <  0] * (ncol - 1) / max(-y[y <  0]), 0)

draw.gradient <- function(x, ys, cols) {
    xs <- rep(x, 2)
    for (i in seq(1, length(ys)-1))
        plot.xy(list(x=xs, y=c(ys[i], ys[i+1])), type="l", col=cols[i])
}

for (i in 1:length(x)) {
    ys <- seq(0, y[i], length.out=idx[i]+1)
    cols <- (if (y[i] >=0) grad.pos else grad.neg)
    draw.gradient(x[i], ys, cols)
}

The result is a decent gradient:

[Graphics output]

I deliberately omitted the scale legend on the right hand side following Allan’s First Law of Happy Graphics: Thou shall not present the same information twice.

For less dense information, you should increase the line width. That is left to the reader. (Hint: it is hard to get just right in base graphics, but lwd <- ceiling(par("pin")[1] / dev.size("in")[1] * dev.size("px")[1] / length(x)) could be a starting point for an approximation. We really need gradient-filled polygons in base R.)

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