[R] childNames for xaxis grob (grid package)

Tobias Verbeke tobias.verbeke at telenet.be
Thu Jul 10 09:27:22 CEST 2008

Hi Paul,

>> Can someone explain why the childNames below
>> gives
>> character(0)
>> instead of the (canonical) names of the children grobs
>> of the xaxis gTree ?
>> [1] "major"  "ticks"  "labels"
> The problem is that you xaxis has an 'at' component of NULL, which means
> that the axis calculates its tick marks on-the-fly when it comes time to
> draw (it has to ask the current viewport what the current scale is).
> This means that there are no grobs representing the major, ticks, and
> labels stored with the xaxis grob on the display list.  These are
> generated just to draw then thrown away.
> If you want to modify the look of the grobs that are drawn (then thrown
> away), you can use the 'edits' component of the xaxis.  This provides an
> edit that will be applied whenever the children are created for drawing.
> Simple example (assuming your code below has been run) ...
>  grid.edit("xa", edits=gEdit("labels", rot=45))

Thank you very much for the detailed explanation. I was trying
to find the grid equivalent of the following base graphics code

atPositions <- axis(2, labels = FALSE)
axis(2, at = atPositions, labels = 10^atPositions)

i.e. use the 'at' to make appropriate labels. I guess there is
no way to do this without passing a (self-made) 'at' to grid.xaxis
in the first place as in

xData <- rpois(5, 12)
yData <- rpois(5, 12)
pushViewport(dataViewport(xData, yData))
grid.points(xData, yData)
atPositions <- grid.pretty(current.viewport()$xscale)
grid.xaxis(name = "xa", at = atPositions, label = 10^atPositions)

Thanks again,

P.S. Maybe the above could be used to populate the
\examples{} section of grid.pretty ?

>> ### minimal example code ###
>> library(grid)
>> pushViewport(plotViewport(c(5,4,4,2)))
>> pushViewport(dataViewport(1:5, 1:5))
>> grid.points(1:5, 1:5)
>> grid.xaxis(name = "xa")
>> grid.get("xa")
>> childNames(grid.get("xa"))

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