Free scales for fill color with geom_tile in ggplot2

Question

I want to set the free scales for the x panel using geom_tile and facet_grid in ggplot2 (see the figure below). To create this shape, I used a for loop and printed in different viewports.

Is there an easy way to create a similar shape?

Thanks for any suggestions.

This is a sample code to recreate my figure.

df <- list(x = 1:5, y = 1:5, xp = 1:3, yp = 1:3)
df <- expand.grid(df)
df$z <- df$xp * 10 + runif(nrow(df))

library(RColorBrewer)
cols <- rev(brewer.pal(11, 'RdYlBu'))
library(ggplot2)
p <- ggplot(df)
p <- p + geom_tile(aes(x, y, fill = z))
p <- p + facet_grid(xp~yp)
p + scale_fill_gradientn(colours = cols)


xp <- unique(df$xp)
library(grid)
for (i in seq(along = xp))
{
    df_i <- df[df$xp == xp[i],]
    p <- ggplot(df_i)
    p <- p + geom_tile(aes(x, y, fill = z))
    p <- p + facet_grid(xp~yp)
    p <- p + scale_fill_gradientn(colours = cols)
    p <- p + theme(
        plot.background = element_rect(fill = 'transparent', colour = 'transparent'),
        strip.background = element_rect(fill = 'transparent', colour = 'transparent'),
        axis.ticks = element_line(colour = 'transparent'))
    p <- p + xlab('') + ylab('')
    if (i > 1)
    {
        p <- p + theme(strip.text.x = element_text(colour = 'transparent'))
    }
    print(p, vp = viewport(0.5, 5/6 - (i - 1) * 0.28, 1, 0.4))
}