Is it possible to put space between stacks in ggpl

I took this example from here:

DF <- read.table(text="Rank F1     F2     F3
1    500    250    50
2    400    100    30
3    300    155    100
4    200    90     10", header=TRUE)

library(reshape2)
DF1 <- melt(DF, id.var="Rank")

library(ggplot2)
ggplot(DF1, aes(x = Rank, y = value, fill = variable)) + 
geom_bar(stat = "identity")

Is it possible to create a stacked bar such as the following graph using ggplot2? I do not want to differentiate stacks by different colors.

EDIT: Based on Pascal's comments,

ggplot(DF1, aes(x = Rank, y = value)) + 
geom_bar(stat = "identity",lwd=2, color="white")

I still have the white borders for the bars.

This is the closest I could get to your example figure. It is not much of an improvement beyond what you've already sorted but puts less of an emphasis on the white bar borders on the grey background.

library(ggplot2)
p <- ggplot(DF1, aes(x = Rank, y = value, group = variable))
p <- p + geom_bar(stat = "identity", position = "stack", lwd = 1.5,
                  width = 0.5, colour = "white", fill = "black")        
p <- p + theme_classic()
p <- p + theme(axis.text.x = element_text(angle = 90, vjust = 0.5))
p

That produces:

If you want to keep the grey background you can find out exactly what shade of grey it is and use that colour for the line while removing the background grids (this is not the right shade).

p <- ggplot(DF1, aes(x = Rank, y = value))
p <- p + geom_bar(stat = "identity", position = "stack", lwd = 1.5,
                  width = 0.5, colour = "grey", fill = "black")        
p <- p + theme(panel.grid = element_blank())
p

An issue with this solution is that very small groups will not be seen (e.g., when Rank = 4 variable F3 = 10; this small value is completely covered by the white bar outline).

Your sample data:

DF1 <- structure(list(Rank = c(1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 
3L, 4L), variable = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 
3L, 3L, 3L, 3L), .Label = c("F1", "F2", "F3"), class = "factor"), 
    value = c(500L, 400L, 300L, 200L, 250L, 100L, 155L, 90L, 
    50L, 30L, 100L, 10L)), row.names = c(NA, -12L), .Names = c("Rank", 
"variable", "value"), class = "data.frame")