Thanks to Kit Baum, a new package -bihist- is available from SSC by typing
ssc install bihist
bihist makes a bihistogram, with frequencies (optionally
fraction/density) of one variable by two categories of another
variable shown above and below the abscissa (the x axis). Which
category's histogram is above the axis and which is below depends on
the ordering of the by() variable. Weights are allowed.