You say "overlap", but any overlapping is a property of
the data rather than of graphical procedure. You
can superimpose histograms, for example like this:
sysuse auto, clear
twoway histogram mpg if foreign, ///
start(10) width(2) bcolor(none) blcolor(red) || ///
histogram mpg if !foreign , ///
start(10) width(2) bcolor(none) blcolor(blue) ///
legend(order(1 "foreign" 2 "domestic") col(1) pos(1) ring(0))
but I find that in general the result is a mess:
1. If distributions overlap, then necessarily one histogram
will partly occlude another. This can be reduced by
for example setting bar colours to invisible, but it cannot
be eliminated. Perceiving the Gestalt is difficult even
for foresters accustoming to seeing the trees for the wood
and the wood for the trees.
2. There is always the minor -- and sometimes the major --
worry of arbitrariness of bin width and origin.
The histogram is 19th century technology: you can do
much better with 1960s technology, namely the quantile-quantile
plot implemented as -qqplot-.
Nick
n.j.cox@durham.ac.uk
Alejandro Delafuente
> am would like to overlap two histograms,
> can anyone tell how
> to do so? The code that I have produced so far is the
> following, but it
> displays two separate histograms with same scale magnitudes:
>
> histogram CONTINUOUS VARIABLE if round!=1, percent
> lcolor(red) ytitle(Percent
> of households) xtitle(???) xlabel(0(.3)1, ticks) title(,
> justification(center))
> note(, justification(center) alignment(top)) legend(off)
> by(round, note
> (Difference in means test significant at 0.01% , size(vsmall)
> justification
> (left)))
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