ratioplot.C

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/// \file
/// \ingroup tutorial_hist_legacy
/// Example displaying two histograms and their ratio. This macro does not use the
/// class TRatioPlot. For ROOT version >= 6.08 TRatioPlot should be used. See
/// the other ratio plots examples in `tutorials/hist`.
///
/// \macro_image
/// \macro_code
///
/// \date May 2021
/// \author Olivier Couet
 
void ratioplot()
{
   // Define two gaussian histograms. Note the X and Y title are defined
   // at booking time using the convention "Hist_title ; X_title ; Y_title"
   TH1F *h1 = new TH1F("h1", "Two gaussian plots and their ratio;x title; h1 and h2 gaussian histograms", 100, -5, 5);
   TH1F("h2", "h2", 100, -5, 5);
   h1->FillRandom("gaus");
   h2->FillRandom("gaus");
 
   // Define the Canvas
   TCanvas *c = new TCanvas("c", "canvas", 800, 800);
 
   // Upper plot will be in pad1
   TPad *pad1 = new TPad("pad1", "pad1", 0, 0.3, 1, 1.0);
   pad1->SetBottomMargin(0); // Upper and lower plot are joined
   pad1->SetGridx();         // Vertical grid
   pad1->Draw();             // Draw the upper pad: pad1
   pad1->cd();               // pad1 becomes the current pad
   h1->SetStats(0);          // No statistics on upper plot
   h1->Draw();               // Draw h1
   h2->Draw("same");         // Draw h2 on top of h1
 
#if ROOT_VERSION_CODE >= ROOT_VERSION(6, 8, 0)
   // Avoid the first label (0) to be clipped.
   TAxis *axis = h1->GetYaxis();
   axis->ChangeLabel(1, -1, -1, -1, -1, -1, " ");
   axis->SetLabelFont(43); // Absolute font size in pixel (precision 3)
   axis->SetLabelSize(15);
#else
   // Do not draw the Y axis label on the upper plot and redraw a small
   // axis instead, in order to avoid the first label (0) to be clipped.
   h1->GetYaxis()->SetLabelSize(0.);
   TGaxis *axis = new TGaxis(-5, 20, -5, 220, 20, 220, 510, "");
   axis->SetLabelFont(43); // Absolute font size in pixel (precision 3)
   axis->SetLabelSize(15);
   axis->Draw();
#endif
 
   // lower plot will be in pad
   c->cd(); // Go back to the main canvas before defining pad2
   TPad *pad2 = new TPad("pad2", "pad2", 0, 0.05, 1, 0.3);
   pad2->SetTopMargin(0);
   pad2->SetBottomMargin(0.2);
   pad2->SetGridx(); // vertical grid
   pad2->Draw();
   pad2->cd(); // pad2 becomes the current pad
 
   // Define the ratio plot
   Clone("h3");
   kBlack);
   h3->SetMinimum(0.8);  // Define Y ..
   h3->SetMaximum(1.35); // .. range
   h3->Sumw2();
   h3->SetStats(0); // No statistics on lower plot
   h3->Divide(h2);
   h3->SetMarkerStyle(21);
   h3->Draw("ep"); // Draw the ratio plot
 
   // h1 settings
   h1->SetLineColor(kBlue + 1);
   h1->SetLineWidth(2);
 
   // Y axis h1 plot settings
   h1->GetYaxis()->SetTitleSize(20);
   h1->GetYaxis()->SetTitleFont(43);
   h1->GetYaxis()->SetTitleOffset(1.55);
 
   // h2 settings
   kRed);
   h2->SetLineWidth(2);
 
   // Ratio plot (h3) settings
   h3->SetTitle(""); // Remove the ratio title
 
   // Y axis ratio plot settings
   h3->GetYaxis()->SetTitle("ratio h1/h2 ");
   h3->GetYaxis()->SetNdivisions(505);
   h3->GetYaxis()->SetTitleSize(20);
   h3->GetYaxis()->SetTitleFont(43);
   h3->GetYaxis()->SetTitleOffset(1.55);
   h3->GetYaxis()->SetLabelFont(43); // Absolute font size in pixel (precision 3)
   h3->GetYaxis()->SetLabelSize(15);
 
   // X axis ratio plot settings
   h3->GetXaxis()->SetTitleSize(20);
   h3->GetXaxis()->SetTitleFont(43);
   h3->GetXaxis()->SetTitleOffset(1);
   h3->GetXaxis()->SetLabelFont(43); // Absolute font size in pixel (precision 3)
   h3->GetXaxis()->SetLabelSize(15);
}