Lines using the R grid package

Question

I am using R with cygwin and am trying to plot some basic graphics. Here is a simple example from one of Paul Murrell's papers:

library(grid)
x <- rnorm(50)
y <- x + rnorm(50, 1, 2)
rx <- range(x)
dx <- diff(rx)
ry <- range(y)
dy <- diff(ry)
max <- max(rx, ry)
min <- min(rx, ry)
r <- c(min(rx, ry), max(rx, ry))
d <- diff(r)
scale <- r + c(-1, 1) * d * 0.05
extscale <- c(min(scale), max(scale) + diff(scale) * 1/3)
lay <- grid.layout(2, 2, 
                   widths = unit(c(3, 1), "inches"), 
                   heights = unit(c(1, 3), "inches"))
vp1 <- viewport(w = unit(4, "inches"), h = unit(4, "inches"), 
                layout = lay, xscale = extscale, yscale = extscale)
grid.newpage()
pushViewport(vp1)
grid.rect()
grid.xaxis()
grid.text("Test", y = unit(-3, "lines"))
grid.yaxis()
grid.text("Retest", x = unit(-3, "lines"), rot = 90)
vp2 <- viewport(layout.pos.row = 2, layout.pos.col = 1, 
                xscale = scale, yscale = scale)
pushViewport(vp2)
grid.lines()
grid.points(x, y, gp = gpar(col = "blue"))

This plots fine. If however I add the following line to the program:

grid.lines(x, y, gp = gpar(col = "red"))

The lines are all over the place. I had expected that lines would connect the points in order. I have had a similar problem with some code I wrote but there the lines plot fine but the points do not.

Would appreciate any help. Thanks.

Answer 1

For some reason, grid.points() and grid.lines() have different "default units". (These are the units -- "native" and "npc" respectively -- that are used when you pass the functions a numeric vector without any associated units.)

args(grid.lines)
# function (x = unit(c(0, 1), "npc"), y = unit(c(0, 1), "npc"), 
#     default.units = "npc", arrow = NULL, name = NULL, gp = gpar(), 
#     draw = TRUE, vp = NULL)     
args(grid.points)
# function (x = stats::runif(10), y = stats::runif(10), pch = 1, 
#     size = unit(1, "char"), default.units = "native", name = NULL, 
#     gp = gpar(), draw = TRUE, vp = NULL) 

The quick solution is to explicitly set the default units for grid.lines() to match those used by grid.points():

grid.lines(x, y, gp = gpar(col = "red"), default.units = "native")