[R] centre two graphs on one plot

[Jim Lemon]

On 06/30/2011 05:37 AM, chris20 wrote:
> Hi,
>
> I am trying to put together a biplot using symbols and different colours
> instead of text as points.
>
> Someone has previously suggested using this code:
>
> PC<- prcomp (iris[,1:4])
> lambda<- PC$sdev * sqrt(nrow(PC$x))
> plot (t(t(PC$x)/lambda),pch=16,col=as.numeric(iris[,5]))
> par (new=T)
> Rot<- t(t(PC$rotation)*lambda)
> XLIM<- c(-max(abs(Rot[,1])),max(abs(Rot[,1])))
> XLIM<- XLIM+(XLIM*0.7)
> plot(Rot,col=4,axes=FALSE,xlim=XLIM,ylim=XLIM,pch="")
> arrows
> (rep(0,nrow(PC$rotation)),rep(0,nrow(PC$rotation)),Rot[,1],Rot[,2],col=4)
> text (Rot[,1:2],rownames(Rot),col=6)
> axis (3)
> axis (4)
>
> But the origin of the arrows does not line up with the origin of the points.
> Can anyone suggest how you would get the two graphs to line up?  I think you
> have to set a ratio between the two sets of axes so that the origin is in
> the centre but I don't know how to do it.
>
Hi Chris,
In order to get the zeros of the two abscissae at the same horizontal 
point, you will have to make both symmetric. Change your third like to:

plot(t(t(PC$x)/lambda),pch=16,col=as.numeric(iris[,5]),
  xlim=c(-0.15,0.15))

and see what happens.

Jim