Equally spaced THist bins

[Ziel_van_brand]

Hi!

I need to plot an histogram so that all the bins visually have the same thickness, although their bins edges are not equally spaced.

This is what I have:

vector<Double_t> x = {0, 1, 2, 3, 4, 5, 10, 15, 30, 60, 90, 180};
vector<Double_t> y = {1.64881E+12, 2.46556E+12, 2.07401E+12, 1.74880E+12, 1.39893E+12, 3.31152E+12, 1.07018E+12, 1.18962E+12, 1.32606E+12, 6.79694E+11, 3.20520E+12};

void angularBinningSTerm () {
	TH1D * h = new TH1D("name", "title", y.size(), (Double_t * ) &x[0]);
	gStyle->SetOptStat(false);
	
	for (size_t i = 0; i < y.size(); i++)
		h->SetBinContent(i + 1, y[i]);
	
	h->Draw("HIST");
}

If you try to run this, you get a graph in which bins don’t have the same “thickness”.

I want them to look like in this sketch:

even if the (bin_upper_edge - bin_lower_edge) is not a constant value.

[RongkunWang]

As far as I know there’s no root module to do this… why don’t you just create a histogram with equal bin width and use SetBinLabel for the Xaxis? Or you can use TText and hard-coded draw the number right at the bin edge

[Ziel_van_brand]

@RongkunWang

Or you can use TText and hard-coded draw the number right at the bin edge

Can you please explain this with an example or can at least give me some more indication on which method I should use?

[RongkunWang]

You can look at this
https://root.cern.ch/doc/master/classTText.html
also useful:
https://root.cern.ch/doc/master/classTAttText.html
https://root.cern.ch/doc/master/classTString.html

Please try to use class reference extensively

// throw away original labels
    h->GetXaxis()->SetLabelOffset(9999);

    TText a;
    a.SetTextColor(kBlack);
    a.SetTextAlign(23);
    a.SetTextFont(41);
    a.SetTextSize(.03);
    for (int i = 0; i < x.size(); i++)
    {
      a.DrawText((double)i, h->GetMinimum() - (h->GetMaximum() - h->GetMinimum()) * .07, TString::Format("%.1f", x[i] ));
    }

RK

[Ziel_van_brand]

Please try to use class reference extensively
I try! I really do. I got lost with your suggestion because I had no clue how to get labels position.

This is what I’ve just done:

void angularBinningSTerm () {
	
	vector<Double_t> x = {0, 1, 2, 3, 4, 5, 10, 15, 30, 60, 90, 180};
	vector<Double_t> y = {1.64881E+12, 2.46556E+12, 2.07401E+12, 1.74880E+12, 1.39893E+12, 3.31152E+12, 1.07018E+12, 1.18962E+12, 1.32606E+12, 6.79694E+11, 3.20520E+12};

	TCanvas * c = new TCanvas("fileName", "window", 600, 400);
	
	TH1D * h = new TH1D("name", "title;x;y", y.size(), 0, y.size());
	gStyle->SetOptStat(false);
	
	for (Int_t i = 0; i < y.size(); i++)
		h->SetBinContent(i + 1, y[i]);
	
	h->GetXaxis()->CenterTitle(true);	
	h->GetXaxis()->SetLabelSize(0);
	h->GetYaxis()->CenterTitle(true);	
	h->GetYaxis()->SetLabelSize(0);
	h->GetYaxis()->SetTitleOffset(0.5);
	h->Draw("HIST");

    TText * a = new TText();
    a->SetTextColor(kBlack);
    a->SetTextAlign(23);
    a->SetTextFont(41);
    a->SetTextSize(.03);
    for (Int_t i = 0; i < x.size(); i++) {
		a->DrawText(h->GetXaxis()->GetBinLowEdge(i+1), h->GetMinimum() - (h->GetMaximum() - h->GetMinimum()) * .07, TString::Format("%.0f", x[i]));
	}
	
	x.clear();
	y.clear();	
}

Thank you!