To draw a "crossed" rectangle of height 2 times larger than its width using the low-level graphics package facilities I call:
xlim <- c(0, 500)
ylim <- c(0, 1000)
plot.new()
plot.window(xlim, ylim, asp=1)
rect(xlim[1], ylim[1], xlim[2], ylim[2])
lines(c(xlim[1], xlim[2]), c(ylim[1], ylim[2]))
lines(c(xlim[1], xlim[2]), c(ylim[2], ylim[1]))
The figure has a nice feature: the aspect ratio is preserved so that if I change the size of the plot window, I get the same height-to-width proportions.
How can I obtain an equivalent result with grid graphics?
You should create a viewport that uses Square Normalised Parent Coordinates,
see ?unit:
"snpc": (...) This is useful for making things which are a proportion of the viewport, but have to be square (or have a fixed aspect ratio).
Here is the code:
library('grid')
xlim <- c(0, 500)
ylim <- c(0, 1000)
grid.newpage() # like plot.new()
pushViewport(viewport( # like plot.window()
x=0.5, y=0.5, # a centered viewport
width=unit(min(1,diff(xlim)/diff(ylim)), "snpc"), # aspect ratio preserved
height=unit(min(1,diff(ylim)/diff(xlim)), "snpc"),
xscale=xlim, # cf. xlim
yscale=ylim # cf. ylim
))
# some drawings:
grid.rect(xlim[1], ylim[1], xlim[2], ylim[2], just=c(0, 0), default.units="native")
grid.lines(xlim, ylim, default.units="native")
grid.lines(xlim, rev(ylim), default.units="native")
The default.units argument in e.g. grid.rect forces the plotting functions
to use the native (xscale/yscale) viewport coordinates.
just=c(0, 0) indicates that xlim[1], ylim[1] denote the bottom-left node
of the rectangle.