[R] control margin when plotting polygons?

[Roger Bivand]

On Fri, 11 May 2007, Shengde Liang wrote:

> 
> Hello everyone,
> 
> I have some difficulties on controling the margin of a plot.
> For example, the following code from the help of maptools:
> 
> library(maptools)
> 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)
> 
> There is a lot of white margin on the bottom and top of the plot and I 
> want to get rid of it. I used option "usr", but it does not make any 
> change.
> 
> plot(xx, border="blue", axes=TRUE, las=1,usr=c(-85,-75,33,39))
> par(usr=c(-85,-75,33,39))
> plot(xx,border="blue",axes=T,las=1)
> 
> Option "ylim" does not work either.

This plot method is more complicated than you can see for geographical 
coordinates, because the y axis is streched. But the real cause of your 
problem is the shape of the plot device. If you compare the output of:

png("default_size.png", pointsize=8)
par(mar=c(3,3,1,1))
plot(xx, border="blue", axes=TRUE, las=1)
dev.off()
png("portrait_size.png", width=300, height=480, pointsize=8)
par(mar=c(3,3,1,1))
plot(xx, border="blue", axes=TRUE, las=1)
dev.off()
png("landscape_size.png", width=480, height=300, pointsize=8)
par(mar=c(3,3,1,1))
plot(xx, border="blue", axes=TRUE, las=1)
dev.off()

you can see that the amount of the plotting area covered by the map can be 
controlled to reduce the white space.

You can also use par(mar=c()) to remove margins, but then possibly without
the axes, or at least watching the effects of las=. The GE_SpatialGrid()  
function also sets up cropping sizes for export of maps in geographical
coordinates as image overlays for Google Earth, and may contain some
ideas.

> 
> Anyone has any idea how to do it?

The answers vary a great deal depending on needs, and are seldom 
completely general. For publication, editing externally may be an option.

Roger

> 
> Thanks a lot.
> 
> Best,
> 
> Shengde Liang
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no