# Re: [R] plotting prcomp

From: Dan Bolser <dan.bolser.r_at_gmail.com>
Date: Fri, 27 Jun 2008 21:43:25 +0200

2008/6/27 Prof Brian Ripley <ripley_at_stats.ox.ac.uk>:
> On Fri, 27 Jun 2008, Prof Brian Ripley wrote:
>
>> On Fri, 27 Jun 2008, Dan Bolser wrote:
>>
>>> Please consider the following PCA example;
>>>
>>> my.df <-
>>> data.frame(A=(x <- rnorm(100,mean=100, sd=10)),
>>> B=(y <- x + rnorm(100,mean=10, sd=10)))
>>>
>>> plot(my.df)
>>>
>>> my.pc <-
>>> prcomp(my.df, center=TRUE, scale=TRUE)
>>>
>>> biplot(my.pc)
>>>
>>> my.x <- (my.pc\$x)[,1]
>>> my.y <- (my.pc\$x)[,2]
>>>
>>> plot(my.x, my.y, type="n")
>>> text(my.x, my.y, labels=1:100)
>>>
>>> Clearly the plots have the same relative values, but I can't work out
how
>>> the second plot is relative to the first. I have been reading the
manual,
>>> but I still can't work it out.
>>>
>>> Can anyone show me the code to generate the data that I see plotted
using
>>> biplot?
>
> I forgot to answer that: just ask R to show you stats:::biplot.prcomp

my.df <-
data.frame(A=(x <- rnorm(100,mean=100, sd=10)),

B=(y <- x + rnorm(100,mean=10, sd=10)))

plot(my.df)

my.pc <-
prcomp(my.df, center=TRUE, scale=TRUE)

biplot(my.pc)

attributes(my.pc)

lam <- my.pc\$sdev

n <- nrow(my.pc\$x)

lam <- lam * sqrt(n)

my.x <- (my.pc\$x)[,1] / lam[1]
my.y <- (my.pc\$x)[,2] / lam[2]

plot(my.x, my.y, type="n")
text(my.x, my.y, labels=1:100)

>>> What am I missing?
>>
>> Reading the references on ?biplot.prcomp, or a good book on the subject
>> (such as MASS or Gower & Hand).
>>
>> A biplot is a specific type of plot and the help page does not spell out
>> the rather mathematical details (and there are several variants).
>>
>>>
>>> Dan.
>>>
>>> [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
>>> R-help_at_r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>> --
>> Brian D. Ripley, ripley_at_stats.ox.ac.uk
>> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
>> University of Oxford, Tel: +44 1865 272861 (self)
>> 1 South Parks Road, +44 1865 272866 (PA)
>> Oxford OX1 3TG, UK Fax: +44 1865 272595
>>
>
> --
> Brian D. Ripley, ripley_at_stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>

```--
hello

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