st: "overlaying" Stacked bar charts

[Devra Golbe <dgolbe@gmail.com>]

Is there a way to "overlay" stacked bar charts?  That is, using the 
citytemp data I can go

webuse citytemp
graph bar tempjan tempjuly, over(region) stack
graph bar  heatdd cooldd, over(region) stack

and get two separate graphs each over region.  What I would like is one 
graph, with the temp bar for each region paired with the corresponding 
degree-day bar.   I guessed the code would be something like the 
following pseudo-code, which results in an error:

twoway (bar tempjan tempjuly ) (bar  heatdd cooldd ), over(region) stack
option over() not allowed
r(198);

I've checked Mitchell's "Visual Guide to Stata Graphics", as well as the 
web and the Statalist, and found no clues.

Thanks,
Devra


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