Bug in geom_raster?

test_raster_ggplot2.R

# I've been testing Hadley's last dev version of ggplot2 - my interest being mainly triggered 
# by the new and shiny geom_raster(), as I'm still convinced a mapping framework based on 
# ggplot2 would just be awesome to see (fast and easy production of very nice looking maps). 
#
# Somehow I managed to test it using a simple coercition of the SpatialPixelsDataFrame objects 
# into data.frames, but for some reasons it seems to be handling the coordinates on a weird way. 
#
# Is that a bug?
#

library(sp)

# Loading demo data
data(meuse.grid)
coordinates(meuse.grid) <- ~x+y
proj4string(meuse.grid) <- CRS("+init=epsg:28992")
gridded(meuse.grid) <- TRUE

# Reference plot
spplot(muse.grid, zcol = "dist")

# Coercition into data.frame
mgdf <- as.data.frame(meuse.grid)
head(mgdf)

# ggplot2 plot
ggplot(mgdf, aes(x = x, y = y)) + geom_raster(aes(fill = dist))

# Looking into the coordinates of mgdf to check this is not as.data.frame()
plot(mgdf$x, mgdf$y, pch = "+", asp = 1)