Area Plots with Intensity Coloring
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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 underappreciated 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:
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 gradientfilled polygons in base R.)
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