r - percentage on y lab in a faceted ggplot barchart?

Question

doing facets in ggplot I would often like the percentage to be used instead of counts.

e.g.

test1 <- sample(letters[1:2], 100, replace=T)
test2 <- sample(letters[3:8], 100, replace=T)
test <- data.frame(cbind(test1,test2))
ggplot(test, aes(test2))+geom_bar()+facet_grid(~test1)

This is very easy but if N is different in facet A compared to facet B, it would be better I think, to compare percentages, in such a way that the each facet sums to 100%.

how would you achieve this?

Hope my question makes sense.

Sincerely.

Answer 1

Here is a within ggplot method, using ..count.. and ..PANEL..:

ggplot(test, aes(test2)) + 
    geom_bar(aes(y = (..count..)/tapply(..count..,..PANEL..,sum)[..PANEL..])) + 
    facet_grid(~test1)

As this is computed on the fly, it should be robust to changes to plot parameters.