Stacking histograms using the lattice library:

library(lattice)
histogram(~X1|V1, data = df0, layout=c(1,2))

Which produces something like this (you’ll need to adjust the layout parameter if there are more factor levels):

Some more options:

histogram(~X1|V1
		, data = df0
		, layout=c(1,2) ##stack them
		, type="count" ##also available are percentage and density 
		, nint=50 ##number of bins
)

Which produces this: