Not understanding the behavior of ..density

Question

In the dataframe below, I would expect the y axis values for density be 0.6 and 0.4, yet they are 1.0. I feel there is obviously something extremely basic that I am missing

Answer 1

As per @Arun's comment:

At the moment, yes and no belong to different groups. To make them part of the same group set a grouping aesthetic:

m <- ggplot(df, aes(x = a , group = 1))   # 'group = 1' sets the group of all x to 1
m + geom_histogram(aes(y = ..density..))