[Giorgio Piccitto]

Dear Nick, thank for your feedback.

This is (more or less) what I was trying to obtain:

local opt1 barw(0.6) lcolor(black) lwidth(vthin)  color(gs15)
local opt2 barw(0.6) lcolor(black) lwidth(vthin)  color(gs10)
local opt3 barw(0.6) lcolor(black) lwidth(vthin)  color(gs5)
local opt4 barw(0.6) lcolor(black) lwidth(vthin)  color(gs1)

twoway bar a id, `opt1' || rbar a ab id, `opt2'  || rbar ab abc id , `opt3' || rbar abc abcd id , `opt4' || ///
   rbar base highx1 id1, barw(0.6) /* color(gs13%40)*/ color(white) lcolor(black) lwidth(vthin) lpattern(dash) sort ///
|| rbar base highx4 id4, barw(0.6) /* color(gs13%40)*/ color(white) lcolor(black) lwidth(vthin) lpattern(dash) sort ///
|| rbar base highx5 id5, barw(0.6) /* color(gs13%40)*/ color(white) lcolor(black) lwidth(vthin) lpattern(dash) sort ///
|| rbar base highx6 id6, barw(0.6) /* color(gs13%40)*/ color(white) lcolor(black) lwidth(vthin) lpattern(dash) sort ///
legend(order(4 "part d" 3 "part c" 2 "part b" 1 "part a") position(6) cols(4) size(small) region(lstyle(none))) ///
ytitle("")  ylabel(-0.12(.02)0.16, angle(horizontal) labsize(small) grid)  xla(1/6, labsize(small)) ///
name(deco_2, replace) ///
    yline(0, lcolor(black) lpattern(solid))

Bars 1, 5 and 6 are the 'overlaid bars of the same height' I was trying to describe (my fault that I was not clear enough, I apologise).

Thanks again for your unvaluable help.

Best and ciao, Giorgio