# r - How are trellis axis limits calculated?

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`.

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`