[R-sig-Geo] Help with rounding legend intervals within a function

[Christopher S. Fowler]

I am producing a large number of maps and wanted to build a function
to set the class interval breaks (and round them to three digits)
within my own function. I found a very helpful interchange at:

https://stat.ethz.ch/pipermail/r-sig-geo/2007-July/002350.html

where the mechanics of how to round legend intervals were set out.
With a few modifications I want to implement that code within my
function.
___________________________________________________
library(cluster)
library(maptools)
library(RColorBrewer)
library(classInt)

#the function accepts a shapefile, a vector of values to plot, a
#number indicating the number of categories, and a title
 myMap<-function(shape,plot.val,ncls=5,title="Generic Title"){
 plotvar <- plot.val
 nclr <- ncls
 plotclr <- brewer.pal(nclr, "BuPu")
 class <- classIntervals(plotvar, nclr, style = "quantile")
 brks <- round(class$brks, 3)
 brks[1]<-(floor(min(class$brks)*1000))/1000  #makes sure lower bound
         #is rounded down so it includes lowest value
 brks[length(brks)]<-(ceiling(max(class$brks)*1000))/1000 #makes sure
         #upper bound is rounded up so it includes highest value
 class <- classIntervals(plotvar, nclr, style = "fixed", fixedBreaks = brks)
 colcode <- findColours(class, plotclr)

 par(mar=c(0.5,0.5,0.5,0.5))
 plot(shape, col = colcode)
 legend("topleft", legend=names(attr(colcode, "table")),
  fill=attr(colcode, "palette"), cex = 0.75, bty = "n")
}

#Run the function
xx <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1],
IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
plot(xx, border="blue", axes=TRUE, las=1)
names(xx)
dat <- cbind(xx$SID74, xx$NWBIR74, xx$BIR79)
fann <- fanny(dat, 3, maxit = 1000)

myMap(xx,fann$membership[,1],ncls=6)
________________________________________________________

Running the function (on R 2.13) produces the error:
"Error in eval(expr, envir, enclos) : object 'brks' not found"

If I run traceback() I get:

5: eval(expr, envir, enclos)
4: eval(mc$...$fixedBreaks)
3: sort(eval(mc$...$fixedBreaks))
2: classIntervals(plotvar, nclr, style = "fixed", fixedBreaks = brks)
1: myMap(xx, fann$membership[, 1], ncls = 6)

what appears to be happening is that the eval expression is using the
main variable environment (if I run the code outside of the function
it works fine, or if I leave an object named brks in the main variable
environment it will see that variable and the function works).

My question is, how do I create the brks variable within the context of
the function so eval can see it, or how do I get the eval expression
to see the brks variable in my function environment? I am leaning
towards the former since I don't want to have to use a custom version
of the classIntervals function if I don't have to.

Thanks for any assistance

-- 
Christopher S. Fowler PhD.
Center for Studies in Demography and Ecology
University of Washington
http://faculty.washington.edu/csfowler