Strange behavior of facet_grid ()

I just came across strange behavior facet_grid()

in ggplot2 0.9 and I am wondering if someone can explain this to me ...

Take the following data frame df

:

var <- sample(c("red", "blue"), 100, replace=TRUE)
group <- sample(c("group1", "group2"), 100, replace=TRUE)
df <- data.frame(var=factor(var), group=factor(group))

      

        
        
        
      

    

What looks like:

  var  group
1 red group2
2 red group1
3 red group1
4 red group2
5 red group2
6 red group1

      

        
        
        
      

    

If I plot the var

marked bar chart , group

I get a rather strange set of y values:

ggplot(data=df, aes(x=var)) + geom_bar() + facet_grid(~group)

      

        
        
        
      

    

It seems strange because the y values ​​seem to be correct if I use facet_wrap

instead facet_grid

:

ggplot(data=df, aes(x=var)) + geom_bar() + facet_wrap(~group)

      

        
        
        
      

    

Also, I can return the correct values ​​with facet_grid

if I inject another dummy variable into the dataframe:

df$tmp <- 1:nrow(df)
ggplot(data=df, aes(x=var)) + geom_bar() + facet_grid(~group)

      

        
        
        
      

    

So, is this some kind of bug or normal behavior that I didn't understand?