How are trellis axis limits calculated?

Question

Say I want to create an ordinary xyplot without explicitly specifying axis limits, then how are axis limits calculated?

The following line of code produces a simple scatter plot. However, axis limits do not exactly range from 1 to 10, but are slightly expanded to the left and right and top and bottom sides (roughly by 0.5).

library(lattice)
xyplot(1:10 ~ 1:10, cex = 1.5, pch = 20, col = "black", 
       xlab = "x", ylab = "y")

Is there any way to determine the factor by which the axes were expanded on each site, e.g. using trellis.par.get? I already tried the following after executing the above-mentioned xyplot command:

library(grid)
downViewport(trellis.vpname(name = "figure"))
current.panel.limits()
$xlim
[1] 0 1

$ylim
[1] 0 1

Unfortunately, the panel limits are returned as normalized parent coordinates, which makes it impossible to obtain the "real" limits. Any suggestions would be highly appreciated!

Update:
Using base-R plot, the data range (and consequently the axis limits) is by default extended by 4% on each side, see ?par. But this factor doesn't seem to apply to 'trellis' objects. So what I am looking for is an analogue to the 'xaxs' (and 'yaxs') argument implemented in par.

Answer 1

Axis limits for xyplot are calculated in the extend.limits function. This function isn't exported from the lattice package, so to see it, type lattice:::extend.limits. Concerning a numeric vector, this function is passed the range of values from the corresponding data (c(1, 10) in this example). The final limits are calculated according to the following equation:

lim + prop * d * c(-1, 1)

• lim are the limits of the data, in this case c(1, 10)
• prop is lattice.getOption("axis.padding")$numeric, which by default is 0.07
• d is diff(as.numeric(lim)), in this case 9

The result in this case is c(0.37, 10.63)

In case you're interested, the call stack from xyplot to extend.limits is

1. xyplot
2. xyplot.formula
3. limits.and.aspect
4. limitsFromLimitList
5. extend.limits