[R-sig-Geo] upper bounds for values in color scale in spplot

[Brad Christoffersen]

Dear R mappers,

I'm just getting started with sp and maptools in R, and I have limited
familiarity with lattice() and how spplot interfaces with lattice() in terms of
commands.  I would like to know how to keep the range of values represented by a
color scale on a series of maps constant.

This is the code I'm running that generates 8 maps across 4 rows and 2 columns. 
You will see that it produces maps that have different upper bounds on the
respective color scales.  I'm interested in having a constant color scale for
each group of 4 maps (1 scale for each column of maps that are plotted) based
on the largest value in the data (or by just prescribing the upper and lower
bounds with constants).

Any help is much appreciated!
Thanks,
Brad C.

library(lattice)
library(sp)
library(maptools)
test <- read.table(##see data below##)
attach(test)
trellis.par.set(sp.theme()) # sets bpy.colors() ramp
seasons <- c("winter", "spring", "summer", "fall")
for(i in 1:length(seasons)){
	coords <- cbind(as.numeric(as.character(test[quarter %in% seasons[i],"lon"])),
as.numeric(as.character(test[quarter %in% seasons[i],"lat"])))
	data <- as.data.frame(test[quarter %in% seasons[i],c("alpha.hat","beta.hat")])
	test.sp <- SpatialPixelsDataFrame(coords, data)
	alpha <- spplot(test.sp, "alpha.hat", scales = list(draw = TRUE))
#		xlab="Degrees longitude", ylab="Degrees latitude", cex=.01)
	beta <- spplot(test.sp, "beta.hat", scales = list(draw = TRUE))
#		xlab="Degrees longitude", ylab="Degrees latitude")
	print(alpha, split = c(1,i,2,4), more=TRUE)
	if(i != 4) print(beta, split = c(2,i,2,4), more=TRUE) else print(beta, split =
c(2,i,2,4), more=FALSE)
	}

#### data to import into "test" above ####
# Exported using write.csv()

"","lon","lat","quarter","alpha.hat","beta.hat"
"1","-92","15","fall",6.65227439310417,56.98796936965
"2","-91","16","fall",5.7887515614357,61.4959602856346
"3","-92","16","fall",8.3574938986946,39.3233936656175
"4","-93","16","fall",6.59178338045291,51.1710742349428
"5","-94","16","fall",4.59110354447175,72.4631790313551
"6","-96","16","fall",3.39120062305053,61.3498455235854
"7","-97","16","fall",3.72637499733858,52.3827509555908
"8","-100","17","fall",3.32679271676835,57.9245201009673
"9","-92","17","fall",7.8180071249829,42.72784154996
"10","-93","17","fall",6.83894383129027,54.4726918059386
"3190","-92","15","winter",5.46418721415736,54.6602669154477
"3191","-91","16","winter",10.2567049399226,26.8445099823192
"3192","-92","16","winter",8.47780749631668,32.3555645381505
"3193","-93","16","winter",8.02490919053258,38.1741888219237
"3194","-94","16","winter",9.26825779141845,31.6804438316389
"3195","-96","16","winter",7.42583760477561,32.4318502183180
"3196","-97","16","winter",7.15366354446019,30.4092071610074
"3197","-100","17","winter",5.67248315575543,44.0629887199626
"3198","-92","17","winter",11.0324295494684,28.5398502105252
"3199","-93","17","winter",12.8600152825316,25.8707202327790
"1064","-92","15","spring",3.61863154993334,165.857349319137
"1065","-91","16","spring",10.1643339841701,35.2421643628567
"1066","-92","16","spring",7.57240369751446,46.9922847457852
"1067","-93","16","spring",6.22249309807625,56.292359117006
"1068","-94","16","spring",4.68274375255797,74.3825475754281
"1069","-96","16","spring",5.53655775167911,44.1263777723619
"1070","-97","16","spring",7.97757426122346,27.6282118121658
"1071","-100","17","spring",6.56565778175741,31.7130717161184
"1072","-92","17","spring",7.6609568130759,55.1152061731614
"1073","-93","17","spring",8.0322728962686,50.598411876406
"2127","-92","15","summer",9.72176297639579,71.2388658845007
"2128","-91","16","summer",16.4747820437274,24.4358645774566
"2129","-92","16","summer",14.0292926267460,27.8787650995959
"2130","-93","16","summer",13.1831296281448,27.3284323593481
"2131","-94","16","summer",6.1805787873833,58.44229189654
"2132","-96","16","summer",5.77117377664185,41.3812553952903
"2133","-97","16","summer",5.98738435242406,35.7155813363363
"2134","-100","17","summer",5.72083279663587,37.0908537701739
"2135","-92","17","summer",9.2083842635031,55.218659463404
"2136","-93","17","summer",13.5244943586978,37.5585163321001