I would suggest you use the command -twoway__histogram_gen- (this is "undocumented", but does have a help file) to generate new variables and then plot them separately, e.g.
sysuse auto, clear
twoway__histogram_gen price, percent gen(h x)
local w=x[2]-x[1]
gen zero=0 if !missing(h)
label var zero "Percent"
twoway rbar h zero x if x<=12000, barw(`w')
Note that this plots all bars from the original histogram (-hist price, percent-) that are centred on a point<=12000 - if you wanted a neat cut at exactly 12000 you would need to specify the -start()- and -width()- options.
David
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