Adding grid unit objects

So the setup is this:

dat <- data.frame(a = c("longnameonthelefthandside"), b = c(sample(10))) 
p <- ggplot(dat, aes(b,a)) + geom_point() + xlab("label")

I can adjust the x axis label by passing a unit object to hjust:

p + opts(axis.title.x=theme_text(size=12,hjust=unit(0.3,"npc")))

But there's something about adding unit objects (possible, according to ?unit) that I'm not grasping:

u1 <- unit(0.5,"npc")
u2 <- unit(0.25,"npc")
p + opts(axis.title.x=theme_text(size=12,hjust=u1+u2))

yields the following error:

Error in grid.C开发者_开发问答all("L_textBounds", as.graphicsAnnot(x$label), x$x, x$y,  : 
Polygon edge not found
In addition: Warning message:
In validDetails.text(x) : NAs introduced by coercion

As further context, I'm trying to piece together a somewhat cryptic workaround via Baptiste here

Probably unit for hjust is implicitly converted to numeric on the way to drawing. So, try this:

grid.lines(c(0.5, 0.5))
grid.text("orzorzorz", y=0.4, hjust=unit(0.25, "npc"))
grid.text("orzorzorz", y=0.5, hjust=unit(0.25, "mm"))
grid.text("orzorzorz", y=0.6, hjust=unit(0.25, "cm"))
grid.text("orzorzorz", y=0.7, hjust=0.25)

and why the u1+u2 induces the error is that:

> c(u1+u2)
$fname
[1] "+"

$arg1
[1] 0.5npc

$arg2
[1] 0.25npc

definitely this cannot be converted into numeric.

so, convertUnit is a workaround, but simply, hjust = c(u1)+c(u2) is sufficient.

Baptiste's code includes a call to convertUnit. Once you put this back into your code, it produces a plot:

p + opts(
  axis.title.x=theme_text(size=12,hjust=convertUnit(u1+u2, "npc", value=TRUE))
)