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

Question

I would like to use ggplot and faceting to construct a series of density plots grouped by a factor. Additionally, I would like to a layer another density plot on each of the facets that is not subject to the constraints imposed by the facet.

For example, the faceted plot would look like this:

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

and then I would like to have the following single density plot layered on top of each of the facets:

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

Furthermore, is ggplot with faceting the best way to do this, or is there a preferred method?

Answer 1

One way to achieve this would be to make new data frame diamonds2 that contains just column price and then two geom_density() calls - one which will use original diamonds and second that uses diamonds2. As in diamonds2 there will be no column clarity all values will be used in all facets.

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

UPDATE - as suggested by @BrianDiggs the same result can be achieved without making new data frame but transforming it inside the geom_density().

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

Another approach would be to plot data without faceting. Add two calls to geom_density() - in one add aes(color=clarity) to have density lines in different colors for each level of clarity and leave empty second geom_density() - that will add overall black density line.

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