r - ggplot2: Is there a way to overlay a single plot to all facets in a ggplot -

i use ggplot , faceting construct series of density plots grouped factor. additionally, layer density plot on each of facets not subject constraints imposed facet.

for example, faceted plot this:

require(ggplot2) ggplot(diamonds, aes(price)) + facet_grid(.~clarity) + geom_density() 

and have following single density plot layered on top of each of facets:

ggplot(diamonds, aes(price)) + geom_density() 

furthermore, ggplot faceting best way this, or there preferred method?

one way achieve make new data frame diamonds2 contains column price , 2 geom_density() calls - 1 use original diamonds , second uses diamonds2. in diamonds2 there no column clarity values used in facets.

diamonds2<-diamonds["price"] ggplot(diamonds, aes(price)) + geom_density()+facet_grid(.~clarity) +       geom_density(data=diamonds2,aes(price),colour="blue") 

update - suggested @briandiggs same result can achieved without making new data frame transforming inside geom_density().

ggplot(diamonds, aes(price)) + geom_density()+facet_grid(.~clarity) +      geom_density(data=transform(diamonds, clarity=null),aes(price),colour="blue") 

another approach plot data without faceting. add 2 calls geom_density() - in 1 add aes(color=clarity) have density lines in different colors each level of clarity , leave empty second geom_density() - add overall black density line.

ggplot(diamonds,aes(price))+geom_density(aes(color=clarity))+geom_density()