#include <TRandom3.h>
#include <TCanvas.h>
#include <TLegend.h>
#include <TLatex.h>
#include <TPave.h>
#include <TH1.h>

void TLegend_example(){

TCanvas* canvas = new TCanvas("canvas","",100,100,600,600);

// Just make some toy histograms
TH1F* hBase = new TH1F("hBase","",100,0,1);
TH1F* h1 = (TH1F*) hBase->Clone("h1");
TH1F* h2 = (TH1F*) hBase->Clone("h2");
TH1F* h3 = (TH1F*) hBase->Clone("h3");

h1->SetLineColor(kRed);
h2->SetLineColor(kGreen);
h3->SetLineColor(kBlue);
h1->SetMarkerColor(kRed);
h2->SetMarkerColor(kGreen);
h3->SetMarkerColor(kBlue);

// Fill them randomly
TRandom3 random;
for(Int_t i=0 ; i<1000 ; i++){
  h1->Fill(random.Gaus(0.2,0.1));
  h2->Fill(random.Gaus(0.5,0.3));
  h3->Fill(random.Gaus(0.8,0.6));
}

// Now make a TLegend and fill it
// This legend will waste space
TLegend legend_bad(0.45,0.8,0.85,0.5);
legend_bad.AddEntry(h1,"h1","pe");
legend_bad.AddEntry(h2,"h2","pe");
legend_bad.AddEntry(h3,"h3","pe");

// Make some TLatex labels
TLatex label_1_bad(0.5,0.90,"label 1");
TLatex label_2_bad(0.5,0.85,"label 2");
TLatex label_3_bad(0.5,0.80,"label 3");
label_1_bad.SetNDC();
label_2_bad.SetNDC();
label_3_bad.SetNDC();

// Draw it all
canvas->cd();
h1->Draw("pe");
h2->Draw("pe:sames");
h3->Draw("pe:sames");
legend_bad.Draw();
label_1_bad.Draw();
label_2_bad.Draw();
label_3_bad.Draw();
canvas->Print("legend_bad.png");

// Now make another TLegend and fill it
// This legend will take up only the space that it needs
TLegend legend_good(0.45,0.8,0.6,0.5);
legend_good.AddEntry(h1,"h1","pe");
legend_good.AddEntry(h2,"h2","pe");
legend_good.AddEntry(h3,"h3","pe");

// Make some TLatex labels
// This time they can be nicely spaced next to the TLegend
TLatex label_1_good(0.65,0.75,"label 1");
TLatex label_2_good(0.65,0.65,"label 2");
TLatex label_3_good(0.65,0.55,"label 3");
label_1_good.SetNDC();
label_2_good.SetNDC();
label_3_good.SetNDC();

// Draw it all
canvas->cd();
h1->Draw("pe");
h2->Draw("pe:sames");
h3->Draw("pe:sames");
legend_good.Draw();
label_1_good.Draw();
label_2_good.Draw();
label_3_good.Draw();
canvas->Print("legend_good.png");

// Everything looks nice now!
// Unfortunately I had to work this all out by hand because I cannot find out
// how much space the text in the TLegend takes up.
// I routinely make many plots, and so do colleagues, so it would be nice to be
// able to automate the allocation of space.
//
// If I try to use TLegend.GetTextSize() it returns 0 (ie let the TLegend set
// it when it gets drawn) so I cannot estimate the size of the text using a
// TLatex object without first setting the size of the text in the TLegend.
// But if I do that then I can't guarantee that the text will fit in the
// TLegend at all!
// Is there a way to esimate how much space the text in a TLegend will take up
// so that I can choose the best size for the TLegend?

}