I have a multi-panel lattice figure. It has an even number of equal-width
columns. I would like to center text across the columns.
The xlab argument often handles this job nicely. But I want a more flexible
solution. (I have many strings and want to position them at different heights,
but always centered across the columns.)
grid.text("Label", x = unit(.5, "npc")) will work when the figure has no
annotations outside the panel. But when it does -- for example, when I use the
scales argument to create row labels -- using grid.text() in this way doesn't
center the text. Here is a minimal example:
dataToPlot <- data.frame(x = 1:2, y = 1:2, panel = c('a', 'b'))
plot1 <- xyplot(
y ~ x | panel,
data = dataToPlot,
layout = c(2,1),
ylab = '',
xlab = '',
scales = list(
x = NULL,
y = list(
draw = TRUE,
labels = c("Label 1", "Label 2"),
at = c(1.8, 1.4))))
print(plot1)
grid.text("XXX", x = unit(.5, "npc"))
In this example, I can fix the problem by seeking the top-level viewport and
then placing a grob in the grid layout:
seekViewport("plot_01.toplevel.vp")
grid.text(
"XXX",
vp = viewport(layout.pos.row=9, layout.pos.col=9:13))
But in that code, centering depends on the specific arguments to layout.pos.row
and layout.pos.col. And when I have figures with more columns or rows, I don't
know what those arguments should be. How can I find out? That is, how can I
learn the layout of "plot_01.toplevel.vp"?
trellis.par.get("layout.widths") and trellis.par.get("layout.heights") are
suggestive of the layout. But I don't have a clear idea of how to use them to
infer the viewport layout, especially when the viewport contains multiple
panels.
I've read through the grid and Lattice documentation, e.g., for
grid.show.layout() and showViewport(). I've also read the Sarkar and Murrell
(1st ed.) books. They're helpful, but I haven't found any passages that speak
to exactly this point.
John
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