Force X axis on both graphs in a facet grid when X values are the same

Question

I have data with about 30 categories for the X axis in two groups for faceting. I will show this with some random data:

dataf <- data.frame(x=c(1:30), A=rnorm(30,20,5), B=rnorm(30,15,0.5))
datam <- melt(dataf, id="x")
ggplot(datam, aes(factor(x), value)) + 
  geom_bar(stat="identity") + 
  facet_grid(variable ~ .)

This is just lovely, except that it would be easier to quickly read off categories on the top grouping if the x axis was reproduced on that graph too. However

ggplot(datam, aes(factor(x), value)) + 
  geom_bar(stat="identity") + 
  facet_grid(variable ~ ., scales="free")

makes no difference to the x axis because, I guess, the values are the same for both groupings.

How can I force the X axis to be reproduced for the top group as well of bars?

Answer 1

Try using facet_wrap instead:

ggplot(datam, aes(factor(x), value)) + 
    geom_bar(stat="identity") + 
    facet_wrap(~variable,nrow = 2,scales = "free")