In 2.1.0: ``` DF <- data.frame(x=rnorm(1000), y=rnorm(1000)) ggplot(DF) + geom_hex(aes(x=x, y=y, fill=..density..)) ``` causes error: ``` Error in eval(expr, envir, enclos) : object 'density' not found ``` But in 2.0.0 (or other older versions), we get (random seeds notwithstanding): 