class TLegend: public TPave, public TAttText

The following example shows how to create a legend. In this example the legend contains a histogram, a function and a graph. The histogram is put in the legend using its reference pointer whereas the graph and the function are added using their names. Note that, because TGraph contructors do not have the TGraph name as parameter, the graph name should be specified using the SetName method.

When an object is added by name, a scan is performed on the list of objects contained in the current pad (gPad) and also in the possible TMultiGraph and THStack present in the pad. If a matching name is found, the coresponding object is added in the legend using its pointer.

{
   TCanvas *c1 = new TCanvas("c1","c1",600,500);
   gStyle->SetOptStat(0);

   TH1F *h1 = new TH1F("h1","TLegend Example",200,-10,10);
   h1->FillRandom("gaus",30000);
   h1->SetFillColor(kGreen);
   h1->SetFillStyle(3003);
   h1->Draw();

   TF1 *f1=new TF1("f1","1000*TMath::Abs(sin(x)/x)",-10,10);
   f1->SetLineColor(kBlue);
   f1->SetLineWidth(4);
   f1->Draw("same");

   const Int_t n = 20;
   Double_t x[n], y[n], ex[n], ey[n];
   for (Int_t i=0;i<n;i++) {
      x[i]  = i*0.1;
      y[i]  = 1000*sin(x[i]+0.2);
      x[i]  = 17.8*x[i]-8.9;
      ex[i] = 1.0;
      ey[i] = 10.*i;
   }
   TGraphErrors *gr = new TGraphErrors(n,x,y,ex,ey);
   gr->SetName("gr");
   gr->SetLineColor(kRed);
   gr->SetLineWidth(2);
   gr->SetMarkerStyle(21);
   gr->SetMarkerSize(1.3);
   gr->SetMarkerColor(7);
   gr->Draw("P");

   leg = new TLegend(0.1,0.7,0.48,0.9);
   leg->SetHeader("The Legend Title");
   leg->AddEntry(h1,"Histogram filled with random numbers","f");
   leg->AddEntry("f1","Function abs(#frac{sin(x)}{x})","l");
   leg->AddEntry("gr","Graph with error bars","lep");
   leg->Draw();

   return c1;
}

Each item in the legend is added using the AddEntry method. This method defines the object to be added (by reference or name), the label associated to this object and an option which a combination of:

• L: draw line associated with TAttLine if obj inherits from TAttLine
• P: draw polymarker associated with TAttMarker if obj inherits from TAttMarker
• F: draw a box with fill associated wit TAttFill if obj inherits TAttFill
• E: draw vertical error bar if option "L" is also specified

As shown in the following example, passing a NULL pointer as first parameter in AddEntry is also valid. This allows to add text or blank lines in a legend.

{
   TCanvas *c2 = new TCanvas("c2","c2",500,300);

   TLegend* leg = new TLegend(0.2, 0.2, .8, .8);
   TH1* h = new TH1F("", "", 1, 0, 1);

   leg->AddEntry(h, "Histogram \"h\"", "l"); 
   leg->AddEntry((TObject*)0, "", "");  
   leg->AddEntry((TObject*)0, "Some text", "");
   leg->AddEntry((TObject*)0, "", "");  
   leg->AddEntry(h, "Histogram \"h\" again", "l");

   leg->Draw();
   return c2;
}