facet_wrap ~var errors but "var" works 

at least we should have a better error message
```r
> ggplot()+
+ facet_wrap(. ~ Species)+
+ geom_point(aes(
+ Sepal.Length, Petal.Length),
+ data=iris)
Erreur dans layout_base(data, vars, drop = drop) : 
  At least one layer must contain all variables used for facetting
> ggplot()+
+ facet_wrap("Species")+
+ geom_point(aes(
+ Sepal.Length, Petal.Length),
+ data=iris)
> 
```