waterfallchart simply uses the lattice package with specific settings to create this type of plot. Consequently, all of the lattice functions that modify plots will work inside of waterfallchart.
You have to set the text parameters of the panel like so:
asTheEconomist(
waterfallchart(value~label, data=rasiel,
groups=subtotal, main="P&L",
panel=function(x, y, ...) {
panel.waterfallchart(x, y, ...);
ltext(x=seq(1,7,1),y=c(75,75,10,1,20,15,14),
labels=c("+150","-170","-20","+10","+18","-2","+6"),
srt=90,font=2,cex=1.5)
}
)
)