# [R] Multiple colors in plots/lookup function

Martin Maechler maechler at stat.math.ethz.ch
Thu Oct 17 15:21:13 CEST 2002

```>>>>> "John" == John Fox <jfox at mcmaster.ca>
>>>>>     on Thu, 17 Oct 2002 07:28:42 -0400 writes:

John> Dear David, At 12:04 AM 10/17/2002 -0400, David
John> Forrest wrote:

>> I'd like to do something like:
>>
>> n<-100 zz<-cbind(rnorm(n),rnorm(n),floor(runif(n)*3+1))
>> colors<-c("red","green","blue")
>>
>> plot(zz,col=colors(zz[3]))
>>
>> and have a matrix of scatterplots colored by class.  The
>> above does not work, of course, but I'm not sure exactly
>> what function I'm looking for.

John> A couple of people have already mentioned that
John> specifying col=colors[zz[,3]] will colour the points
John> in a scatterplot (not a scatterplot matrix) by the
John> values of zz[,3].

yes, even in a scatterplot matrix.
Try a very slight variation of the above :

n <- 100
zz <- data.frame(Z1 = rnorm(n), Z2 = rnorm(n), Ufact = floor(runif(n)*3+1))
colors <- c("red","green","blue")
plot(zz, col = colors[zz[, 3]])

i.e., the only practical difference is that I used
cbind()

--> which changes things:  The plot() method for a data.frame calls
pairs() which *is* the same as a scatterplot matrix.

John> 	Since there are just two other
John> columns (the third column of zz is ignored in the
John> first argument to plot), you get a scatterplot, not a
John> scatterplot matrix, and it's hard to see how a
John> scatterplot matrix would be relevant.

indeed!

John> If, however, you want a scatterplot matrix of pairwise
John> plots for three or more variables coloured by the
John> levels of a factor, the scatterplot.matrix function in
John> the car package should do what you want.

I don't see why pairs()  {or as mentioned,  plot( ) for a data.frame}
shouldn't be sufficient.

Regards,
Martin Maechler <maechler at stat.math.ethz.ch>	http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum  LEO C16	Leonhardstr. 27
ETH (Federal Inst. Technology)	8092 Zurich	SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1228			<><
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```