How to convert array dim names to expressions for plotting?

Hi,

I would like to use the dim names of an array as labels in a plot. In the case of character labels, this is no problem. However, if I would like to pass expressions through array dim names, they only appear as character labels in the plot and are not evaluated as expressions (see the minimal example below). How can this be done?

Cheers,

Marius


require(grid)

tau <- c(0.25, 0.75)
alpha <- c(0.95, 0.99, 0.999)
a <- array(1:6, dim=c(2,3), dimnames=list(tau=tau, alpha=alpha))

taulabs <- as.expression(lapply(tau, function(t) bquote(tau==.(t))))
dimnames(a)$tau <- taulabs # does not create expressions
dimnames(a) # => no expressions
dimnames(a)$tau[[1]] <- taulabs[1]
dimnames(a)$tau[[2]] <- taulabs[2]
dimnames(a) # => now characters

gl <- grid.layout(1, 2, default.units="inch", widths=c(2,2), heights=c(2,2))
## grid.show.layout(gl)
pushViewport(viewport(layout=gl))

pushViewport(viewport(layout.pos.row=1, layout.pos.col=1))
grid.rect(gp=gpar(col=NA, fill="gray80"))
grid.text(dimnames(a)[["tau"]][1], x=0.5, y=0.5)
upViewport()

pushViewport(viewport(layout.pos.row=1, layout.pos.col=2))
grid.rect(gp=gpar(col=NA, fill="gray50"))
grid.text(dimnames(a)[["tau"]][2], x=0.5, y=0.5)
upViewport()