facet_wrap: How to add y axis to every individual graph when scales="free_x"?

Question

The following code

library(ggplot2)
library(reshape2)

m=melt(iris[,1:4])

ggplot(m, aes(value)) + 
  facet_wrap(~variable,ncol=2,scales="free_x") +
  geom_histogram()

produces 4 graphs with fixed y axis (which is what I want). However, by default, the y axis is only displayed on the left side of the faceted graph (i.e. on the side of 1st and 3rd graph).

What do I do to make the y axis show itself on all 4 graphs? Thanks!

EDIT: As suggested by @Roland, one could set scales="free" and use ylim(c(0,30)), but I would prefer not to have to set the limits everytime manually.

@Roland also suggested to use hist and ddply outside of ggplot to get the maximum count. Isn't there any ggplot2 based solution?

EDIT: There is a very elegant solution from @babptiste. However, when changing binwidth, it starts to behave oddly (at least for me). Check this example with default binwidth (range/30). The values on the y axis are between 0 and 30,000.

library(ggplot2)
library(reshape2)

m=melt(data=diamonds[,c("x","y","z")])

ggplot(m,aes(x=value)) + 
  facet_wrap(~variable,ncol=2,scales="free") +
  geom_histogram() +
  geom_blank(aes(y=max(..count..)), stat="bin")

And now this one.

ggplot(m,aes(x=value)) + 
  facet_wrap(~variable,scales="free") +
  geom_histogram(binwidth=0.5) +
  geom_blank(aes(y=max(..count..)), stat="bin")

The binwidth is now set to 0.5 so the highest frequency should change (decrease in fact, as in tighter bins there will be less observations). However, nothing happened with the y axis, it still covers the same amount of values, creating a huge empty space in each graph.

[The problem is solved... see @baptiste's edited answer.]

Answer 1

Is this what you're after?

ggplot(m, aes(value)) + 
  facet_wrap(~variable,scales="free") +
  geom_histogram(binwidth=0.5) +
  geom_blank(aes(y=max(..count..)), stat="bin", binwidth=0.5)

Answer 2

ggplot(m, aes(value)) + 
  facet_wrap(~variable,scales="free") +
  ylim(c(0,30)) +
  geom_histogram()