R: Making common legend and colour scheme for multiple shapefiles - r

Question

A sample data first:
library(raster)
# download a sample shape file
shape.file <- getData('GADM', country='FRA', level=2)
ID_2 <- rep(shape.file#data$ID_2,times=2)
group.id <- rep(c(100,200),each=96)
set.seed(1)
cat1<- runif(192,min=0,max=100)
set.seed(2)
cat2<- runif(192,min=0,max=100)
set.seed(3)
cat3<- runif(192,min=0,max=100)
dat <- as.data.frame(cbind(ID_2,group.id,cat1,cat2,cat3))
# extract the data for group.id = 100 and group.id = 200 in two seprate df
group.id.100 <- dat[dat$group.id==100,]
group.id.200 <- dat[dat$group.id==200,]
# merge with shape file
merge.shp.100 <- merge(shape.file,group.id.100, by="ID_2")
merge.shp.200 <- merge(shape.file,group.id.200, by="ID_2")
# plot them together
par(mfrow=c(3,2))
plot(merge.shp.100,col=merge.shp$cat1,main="Group.id.100,cat1")
plot(merge.shp.100,col=merge.shp$cat2,main="Group.id.100,cat2")
plot(merge.shp.100,col=merge.shp$cat3,main="Group.id.100,cat3")
plot(merge.shp.200,col=merge.shp$cat1,main="Group.id.200,cat1")
plot(merge.shp.200,col=merge.shp$cat2,main="Group.id.200,cat2")
plot(merge.shp.200,col=merge.shp$cat3,main="Group.id.200,cat3")
I want to insert a common legend and colour scheme for all the 6 figures.
For example, the legend should go from minimum = min(c(cat1,cat2,cat3)) to max(c(cat1,cat2,cat3))
for all the six figures. Similarly, a value of 50 should have the same colour in all the 6 figures.
Thank you

Answer 1

You can use spplot
Example data:
library(raster)
s <- getData('GADM', country='FRA', level=2)[, c('GID_1', 'NAME_1', 'GID_2', 'NAME_2')]
set.seed(1)
m <- matrix(runif(96*6, 0, 100), nrow=96, ncol=6)
vars <- paste0("cat", 1:6)
colnames(m) <- vars
d <- data.frame(GID_2 = s$GID_2, m)
sd <- merge(s, d)
Use spplot:
spplot(sd, vars)
With base plot, you could do something like
brks <- seq(0,100,20)
par(mfrow=c(2,3), mai=c(0,0,0.5,0))
cols = rainbow(length(brks))
for (v in vars) {
cuts <- cut(data.frame(sd)[, v], brks)
plot(s, col=cols[cuts], main=v)
}