Filling rows when using facet_grid and geom_raster in ggplot2

Let say I did four measurements/experiments (M1..M4) producing intensity values which vary across 5 locations (loc_1...5). I observed various classes of elements (n=7). Now I would like to summarize the results into a single diagram using facet_grid and geom_raster from ggplot2. I end up with the diagram below (see image).

Now the question is whether there is a simple solution to force rows to fill the space in each facet (i.e to drop unused rows in each panel).

Thank you

rm(list=ls())
library(ggplot2)
library(reshape2)

set.seed(123)

# let's create a fake dataset
nb.mesure <- 4
n.row <- 200
n.col <- 5
nb.class <- 7


d <- matrix(round(runif(n.row * n.col),2), nc=n.col) 
colnames(d) <- sprintf("Loc_%02d", 1:5)

# These strings will be the row names of each heatmap
# in the subsequent facet plot
elements <- sample(replicate(n.row/2, 1:100))

# let's create a data.frame d
d <- data.frame(d, 
                mesure = sort(rep(c("M1","M2","M3", "M4"), n.row/4)),
                elements= elements,
                class=sample(nb.class, 
                             length(elements), 
                             rep=T,
                             prob = seq(0.01, 0.25, length.out=7))
                             )



# Data are melt
dm <- melt(d, id.var=c( "mesure", "elements", "class"))
colnames(dm) <- c("mesure","elements", "class", "pos", "intensity")

# Plotting
p <- ggplot(dm, aes(x = pos, y = elements, fill = intensity))
p <- p + geom_raster()                                        
p <- p + facet_grid(mesure~class , scales = "free", space="free_y")
p <- p + theme_bw()
p <- p + theme(text = element_text(size=8))
p <- p + theme(text = element_text(family = "mono", face = "bold"))
p <- p + theme(axis.text.y = element_blank(), 
               axis.ticks.y=element_blank(),
               axis.text.x = element_text(colour="grey20",
                                           size=6,angle=45,
                                        vjust = 0.3))
print(p)

If your goal is to introduce NA values (or something else) for all missing combinations of mesure, elements, class and pos you can use the complete function from the tidyr package like so:

library(tidyr)

dm <- complete(dm, mesure, elements, class, pos, fill = list(intensity = NA))

UPDATE

In case you want to have the non-NA values expand to fill each facet you have to move away from facet_grid and switch to facet_wrap.

p <- p + facet_wrap(mesure~class , scales = "free_y", nrow = 4)