how to specify the size of device so that it fits a SpatialPolygonsDataFrame plot exactly

 Le 28/07/2019 ? 17:08, Roger Bivand a ?crit?: This feels like a section in
 ASDAR, ch. 3, and code chunks 24-27 in
 https://asdar-book.org/book2ed/vis_mod.R. Could you please try that first
 by way of something reproducible?

Le 28/07/2019 ? 17:15, Patrick Giraudoux a ?crit?:

 Ups... How? can I have missed this chapter of my bible ;-) ? (must admit I
 had been on google first). Will re-read it carefully and come back to the
 list with a solution or a reproducible example, indeed.

OK. Read it again, I was not totally lost. Here is a reproducible example. To 
ease reproducibility with simple objects, I will use two bounding boxes.? 
bbChina in WGS84, bbChinaUTM47 in UTM47. I want a window fitting the WGS84, 
and you'll see I get it through a strange way.

bbChina <- new("SpatialPolygons", polygons = list(new("Polygons", Polygons = 
list( new("Polygon", labpt = c(104.8, 35.95), area = 2372.28, hole = FALSE, 
ringDir = 1L, coords = structure(c(73, 73, 136.6, 136.6, 73, 17.3, 54.6, 
54.6, 17.3, 17.3), .Dim = c(5L, 2L)))), plotOrder = 1L, labpt = c(104.8, 
35.95), ID = "1", area = 2372.28)), plotOrder = 1L, bbox = structure(c(73, 
17.3, 136.6, 54.6), .Dim = c(2L, 2L), .Dimnames = list(c("x", "y"), c("min", 
"max"))), proj4string = new("CRS", projargs = "+init=epsg:4326 +proj=longlat 
+datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"))

bbChinaUTM47 <- new("SpatialPolygons", polygons = list(new("Polygons", 
Polygons = list( new("Polygon", labpt = c(856106.391943348, 4317264.60126758 
), area = 30651262771540.1, hole = FALSE, ringDir = 1L, coords = 
structure(c(-2331000.09677063, -2331000.09677063, 4043212.88065733, 
4043212.88065733, -2331000.09677063, 1912947.1678777, 6721582.03465746, 
6721582.03465746, 1912947.1678777, 1912947.1678777), .Dim = c(5L, 2L)))), 
plotOrder = 1L, labpt = c(856106.391943348, 4317264.60126758), ID = "1", area 
= 30651262771540.1)), plotOrder = 1L, bbox = structure(c(-2331000.09677063, 
1912947.1678777, 4043212.88065733, 6721582.03465746), .Dim = c(2L, 2L), 
.Dimnames = list(c("x", "y"), c("min", "max"))), proj4string = new("CRS", 
projargs = "+init=epsg:4326 +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 
+towgs84=0,0,0"))

Then let's go:

Example #1: here, being straightforward we get two indesirable white strips 
on the sides:

width1<-max(bbox(bbChina)[1,])-min(bbox(bbChina)[1,])
height1<-max(bbox(bbChina)[2,])-min(bbox(bbChina)[2,])
ratio<-width1/height1
ratio
windows(height=8,width=8*ratio)
par(mar=c(0,0,0,0))
plot(bbChina,col="grey",xaxs='i', yaxs='i')
dev.off()

Example #2: computing the ratio on UTM47, but plotting WGS84 (strange), I get 
a better fit but with still two small white strips up and down.

width1<-max(bbox(bbChinaUTM47)[1,])-min(bbox(bbChinaUTM47)[1,])
height1<-max(bbox(bbChinaUTM47)[2,])-min(bbox(bbChinaUTM47)[2,])
ratio<-width1/height1
ratio
windows(height=8,width=8*ratio)
par(mar=c(0,0,0,0))
plot(bbChina,col="grey",xaxs='i', yaxs='i') # no data range extention (xaxs 
and yaxs parameter)

dev.off()

Example #3: multiplying the ratio by 1.04, I get a good fit

width1<-max(bbox(bbChinaUTM47)[1,])-min(bbox(bbChinaUTM47)[1,])
height1<-max(bbox(bbChinaUTM47)[2,])-min(bbox(bbChinaUTM47)[2,])
ratio<-width1/height1
ratio
windows(height=8,width=8*ratio*1.04)
par(mar=c(0,0,0,0))
plot(bbChina,col="grey",xaxs='i', yaxs='i')

dev.off()

Looks like the issue has something to do with the way CRS are handled when 
plotting objects, mmmh ? Tricky isn't it ?