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What's box() (exactly) doing?

3 messages · Marius Hofert, David Winsemius, Jim Lemon

Marius Hofert
# 
Hi,

I would like to replicate the behavior of box() with rect() (don't ask why).
However, my rect()angles are always too small. I looked a bit into the
internal C_box but
couldn't figure out how to solve the problem. Below is a minimal
working (and a slightly bigger) example.

Cheers,
Marius

## MWE
plot(NA, type = "n", ann = FALSE, axes = FALSE, xlim = 0:1, ylim = 0:1)
rect(xleft = 0, ybottom = 0, xright = 1, ytop = 1, col = "grey80") #
should match box()
box()

## Extended example

## Basic plot
my_rect <- function()
{
    plot(NA, type = "n", ann = FALSE, axes = FALSE, xlim = 0:1, ylim = 0:1)
    rect(xleft = 0, ybottom = 0, xright = 1, ytop = 1, col = "grey80")
# should match box()
    box()
}

## Layout
lay <- matrix(0, nrow = 3, ncol = 3, byrow = TRUE)
lay[1,1] <- 1
lay[2,1] <- 2
lay[2,2] <- 3
lay[2,3] <- 4
lay[3,3] <- 5
layout(lay, heights = c(1, 10, 1), widths = c(10, 1, 10))
layout.show(5) # => no space between rectangles; calls box() to draw the boxes

## Fill layout
par(oma = rep(0, 4), mar = rep(0, 4))
my_rect()
my_rect()
my_rect()
my_rect()
my_rect()
## => spaces between rectangles => why?/how to avoid?
David Winsemius
# 
Are you aware that the range of the plot area is expanded by a factor of something on the order of 1.04 so that data "points" will appear whole? If my memory serves you can call plot with the parameter plot=FALSE and then recover the par(usr) values for the plot area.
Jim Lemon
# 
Hi Marius,
There are a few things that are happening here. First, the plot area
is not going to be the same as your x and y limits unless you say so:

# run your first example
par("usr")
[1] -0.04  1.04 -0.04  1.04

# but
plot(NA, type = "n", ann = FALSE, axes = FALSE,
 xlim = 0:1, ylim = 0:1,xaxs="i",yaxs="i")
box()
rect(xleft = 0, ybottom = 0, xright = 1, ytop = 1, col = "grey80")
par("usr")
[1] 0 1 0 1

Second, the "rect" function is automatically clipped to the plot area,
so you may lose a bit at the edges if you don't override this:

par(xpd=TRUE)
rect(...)
par(xpd=FALSE)

Finally your second example simply multiplies the first problem by
specifying a layout of more than one plot. Applying the "xaxs" and
"yaxs" parameters before you start plotting will fix this:

par(xaxs="i",yaxs="i")

Jim

On Fri, Jun 24, 2016 at 12:29 PM, Marius Hofert
<marius.hofert at uwaterloo.ca> wrote: