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

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.]