R help - Sep 2008 - How to put given values in lower triangle of splom-plot?

Dear R-experts,

I have found a splom-modification online which is given below. This  
works perfectly, but I would like to have a matrix of given  
correlation values to be used in the lower triangular part  
(lower.panel) of the splom-plot instead of calculated correlation  
values. Here is the matrix I would like to use (it can be any other  
convenient data structure):

mymat=matrix(0,nrow=3,ncol=3)
mymat[1,2]=0.2
mymat[1,3]=0.2
mymat[2,3]=0.5

If one can determine inside the lower.panel-function which columns of  
"data" are used for the current pairwise scatter plot, it should be  
easy to get the correct entries of "mymat", but how is this achieved?
It would then be also possible to build a data structure of locations  
so that one can place the entries wherever convenient inside the gray  
rectangles.

Best regards.

Marius

#splom modification example
library(lattice)
data1=runif(100)
data2=data1+runif(100)
data3=data2+runif(100)
data=cbind(data1,data2,data3)
splom(~data[,1:3],upper.panel=panel.splom,
       lower.panel=function(x,y,...){
					panel.fill(col=gray(500:1000/1000)[round(501-500*cor(x,y))])
           panel.text(mean(x),mean(y),round(cor(x,y),2))
       },
cex=0.2,aspect=1,pscales=0,varnames=c("Component 1","Component  
2","Component 3"),xlab="",ylab="")

On Sun, Sep 21, 2008 at 2:11 AM, Hofert Marius <m_hofert at web.de>
wrote:
> Dear R-experts,
>
> I have found a splom-modification online which is given below. This works
> perfectly, but I would like to have a matrix of given correlation values to
> be used in the lower triangular part (lower.panel) of the splom-plot
instead
> of calculated correlation values. Here is the matrix I would like to use
(it
> can be any other convenient data structure):
>
> mymat=matrix(0,nrow=3,ncol=3)
> mymat[1,2]=0.2
> mymat[1,3]=0.2
> mymat[2,3]=0.5
>
> If one can determine inside the lower.panel-function which columns of
"data"
> are used for the current pairwise scatter plot, it should be easy to get
the
> correct entries of "mymat", but how is this achieved? It would
then be also
> possible to build a data structure of locations so that one can place the
> entries wherever convenient inside the gray rectangles.
You do not have access to the row and column numbers inside the panel
function. The intention is to have the `panel' function handle pairs
of variables, and the `superpanel' function to handle the global data
frame; so one approach could be to replace the default superpanel
function:


mymat <- mymat + t(mymat) # to fill in lower diagonal

splom(~data[,1:3],
      superpanel = function(z, ...) {
          mymat.df <-
              data.frame(vals = as.vector(mymat),
                         row = as.vector(row(mymat)),
                         col = as.vector(col(mymat)))
          mymat.df <- subset(mymat.df, col < row)
          with(mymat.df,
           {
               panel.rect(x = row, y = col, width = 1, height = 1,
                          col = gray(500:1000/1000)[round(501 - 500 * vals)])
               panel.text(x = row, y = col, labels = round(vals, 2))
           })
          panel.pairs(z, upper.panel = panel.splom,
                      lower.panel = function(...) {},
                      ...)
      },
      cex=0.2,
      aspect=1,
      pscales=0,
      varnames=c("Component 1","Component 2","Component
3"),
      xlab="",ylab="")


This uses one undocumented fact (which I will shortly document),
namely, that the coordinate system set up before 'superpanel' is
called has both x- and y-limits set to c(0.5, ncol(z)+0.5).

-Deepayan