Plotting 2 histogram on top and bottom part of a single plot

I want to draw two histogram on same plot window but placed vertically. One such example can be found at https://blogs.sas.com/content/graphicallyspeaking/files/2013/11/MirrorHistogramVert.png

Below is my code

library(ggplot2)
set.seed(1)
dat = rbind(data.frame('val' = rnorm(100), 'met' = 'Metric1'), data.frame('val' = rt(100, 2), 'met' = 'Metric12'))

ggplot(dat, aes(x = val, y = met, color = met)) + geom_histogram()

However I am getting below error

Error in `geom_histogram()`:
! Problem while computing stat.
ℹ Error occurred in the 1st layer.
Caused by error in `setup_params()`:
! `stat_bin()` must only have an x or y aesthetic.
Run `rlang::last_trace()` to see where the error occurred.

I also want to add probability curve for Normal distribution for both histogram.

Any suggestion what could be the right approach will be very helpful

Solution

You could do two separate layers for the two groups, using y = -after_stat(count) in the second layer (the default for geom_histogram is y = after_stat(count).

Ensure the breaks for both layers are the same so that the bins line up.

ggplot(dat, aes(x = val, fill = met)) + 
  geom_histogram(data    = dat[dat$met == 'Metric1',], 
                 breaks  = seq(-40, 10, 0.5),
                 colour  = "black",
                 alpha   = 0.3) +
  geom_histogram(data    = dat[dat$met == 'Metric12', ],
                 breaks  = seq(-40, 10, 0.5),
                 mapping = aes(y = -after_stat(count)),
                 colour  = "black",
                 alpha   = 0.3) +
  scale_fill_manual(values = c('blue3', 'red3')) +
  coord_cartesian(xlim = c(-10, 10)) +
  labs(y = 'count') +
  theme_bw(20) +
  theme(panel.border = element_blank())