Re: [R] superimposing histograms con't

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Fri 30 Jun 2006 - 01:37:47 EST

>>>>> "Bill" == Bill Shipley <bill.shipley@usherbrooke.ca> >>>>> on Wed, 28 Jun 2006 15:53:35 -0400 writes:

    Bill> Earlier, I posted the following question:

    Bill> I want to superimpose histograms from three
    Bill> populations onto the same graph, changing the shading
    Bill> of the bars for each population. After consulting the
    Bill> help files and the archives I cannot find out how to
    Bill> do this (seemly) simple graph. To be clear, I want

    Bill> - a single x axis (from -3 to 18)
    Bill> - three groups of bars forming the histograms of each population
    Bill>   (they will not overlap much, but this is a detail)
    Bill> - the bars from each histogram having different     Bill> shadings or other visually distinguishing features.  
    Bill> Gabor Grothendieck [ggrothendieck@gmail.com] pointed
    Bill> to some code to to this but I have found another way
    Bill> that works even easier.
 

    Bill> hist(x[sel1],xlim=c(a,b),ylim=c(A,B))  - this plots the histogram for the
    Bill> first group (indexed by sel1) but with an x axis and a y axis that spans the     Bill> entire range.  

    Bill> par(new=T) - to keep on the same graph  

    Bill> hist(x[sel2],main=Null,xlab=NULL,ylab=NULL,axes=F) -superimposes the second     Bill> histogram  

    Bill> par(new=T) - to keep on the same graph  

    Bill> hist(x[sel3],main=Null,xlab=NULL,ylab=NULL,axes=F) -superimposes the third     Bill> histogram  

Hmm, the above does not work (because of 'Null' instead of 'NULL') but even if you fix that, I'm pretty sure you get pretty wrong plots.

par(new = TRUE) is quite often a bad choice. In the present case, plot 2 and 3 use different coordinate systems than plot 1.

The "correct" solution -- apart from using lattice::histogram() with the correct 'superpose' panel is really to make use of the fact that

  1. hist() returns a "histogram" (S3) object which is plotted by
  2. plot.histogram() which has a nice help page even though the funcion is namespace-hidden.

Here's a reproducible solution --- with quite a bit of extra code in order to show two ``colorizations'' for overplotting, in particular one with ``transparent colors'' :

##MM: construct reproducible example data set.seed(1)
x <- c(rnorm(50), (rnorm(60) +3.5), (rnorm(40) +3.5 + 3)) str(grouping <- factor(c(rep(1,50), rep(2,60), rep(3,40)))) (n.gr <- length(table(grouping))) # 3

xr <- range(x)
## Compute all three histograms objects but without plotting:
histL <- tapply(x, grouping, hist, plot = FALSE) maxC <- max(print( sapply(lapply(histL, "[[", "counts"), max)) ) # 14 15 15

pdf("3histo.pdf", version = "1.4") # >= 1.4 is needed
## or try the default x11()

if((TC <- transparent.cols <- .Device %in% c("pdf", "png"))) {

    cols <- hcl(h = seq(30, by=360 / n.gr, length = n.gr), l = 65,

                alpha = 0.5) ## << transparency
} else {

    h.den <- c(10, 15, 20)
    h.ang <- c(45, 15, -30)
}

## Plots the histogram for the
## first group (indexed by sel1) but with an x axis and a y axis that spans the
## entire range.
if(TC) {

    plot(histL[[1]], xlim = xr, ylim= c(0, maxC), col = cols[1],

         xlab = "x", main = "3 Histograms of 3 Selections from 'x'") } else

    plot(histL[[1]], xlim = xr, ylim= c(0, maxC), density = h.den[1], angle = h.ang[1],

         xlab = "x", main = "Histogram of 3 selections from 'x'")
## Using density instead of color make these *add*

if(!transparent.cols) {

    for(j in 2:n.gr)

        plot(histL[[j]], add = TRUE, density = h.den[j], angle = h.ang[j]) } else { ## use semi-translucent colors (available only for PDF >= 1.4 and PNG devices):

    for(j in 2:n.gr)

        plot(histL[[j]], add = TRUE, col = cols[j]) }

if(.Device == "pdf") { ## you have set it above   dev.off()
  system("gv 3histo.pdf &")## or use 'acroread' ; 'xpdf' does not show transparent colors... }

1 ##--- Martin Maechler, ETH Zurich



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