What you see is that class “feature”. Here you are the piece of the THStack::Paint(Option_t *option) method that explains your observation. Pay your attention to the hardcoded:
h1->SetFillColor(1000);
h1->SetFillStyle(1001);
h1->Paint(loption);
for (Int_t i=0;i<nhists;i++) {
if (strstr(lnk->GetOption(),"same")) {
sprintf(loption,"%s%s",noption,lnk->GetOption());
} else {
sprintf(loption,"%ssame%s",noption,lnk->GetOption());
}
h1 = (TH1*)fStack->At(nhists-i-1);
if (i>0) {
// Erase before drawing the histogram
h1col = h1->GetFillColor();
h1fill = h1->GetFillStyle();
h1->SetFillColor(1000);
h1->SetFillStyle(1001);
h1->Paint(loption);
static TClassRef clTFrame = TClass::GetClass("TFrame",kFALSE);
TAttFill *frameFill = (TAttFill*)clTFrame->DynamicCast(TAttFill::Class(),gPad->GetFrame());
h1->SetFillColor(frameFill->GetFillColor());
h1->SetFillStyle(frameFill->GetFillStyle());
h1->Paint(loption);
h1->SetFillColor(h1col);
h1->SetFillStyle(h1fill);
}
h1->Paint(loption);
lnk = (TObjOptLink*)lnk->Prev();
} I slightly changed your macro (see attachment) and re-run it with the QtRoot plugin (see the thread Transparent histogram fill style for some details)
THStackOverlay.hash.C (2.09 KB)
THStackOverlay.4.QtRoot.C (2.04 KB)