How to set the minimum value of a histogram to zero

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2

Try:

signal1D2->Scale(1. / signal1D2->GetBinContent(1)); // (1) or maybe better (2)
signal1D3->Scale(1. / signal1D3->GetBinContent(1)); // (1) or maybe better (2)
signal1D4->Scale(1. / signal1D4->GetBinContent(1)); // (1) or maybe better (2)

or:

signal1D2->Scale(1. / signal1D2->Integral());
signal1D3->Scale(1. / signal1D3->Integral());
signal1D4->Scale(1. / signal1D4->Integral());

or:

signal1D2->Scale(1. / signal1D2->GetEntries());
signal1D3->Scale(1. / signal1D3->GetEntries());
signal1D4->Scale(1. / signal1D4->GetEntries());