Shading under a TGraph with color as a function of another variable

[kvetter]

Hello,

I have two TGraphs. One contains the amplitude of a certain quantity as a function of frequency, and the other contains the phase of the same quantity, as a function of frequency with the same frequency axis. I would like to visualize this as a single plot where the Y axis of the graph represents the amplitude and the color underneath the graph represents the phase. Here is a link to a plot which represents something similar to what I am hoping to make: https://i.stack.imgur.com/3RKUD.png Hopefully the problem statement is clear. I appreciate any advice anyone may have.

Unfortunately I cannot share the TGraphs I am referencing. I am using ROOT 6 locally, though the machine on which the graphs are made uses ROOT 5.

Thank you,
Kenny

[couet]

So the colors below are vertical bars and the color of each bar is defined according to its content ? (assuming the plot below is more likely an histogram) ?

[dastudillo]

You can create a 2D histogram (maybe TH2F), where the x bins are the frequencies and y are the intensities.
Then you fill all the y bins of a given x with the same phase using SetBinContent, going from y=0 to y=intensity; or you could fill the bins of y=intensity with a special value (colour) to highlight the envelope curve. Then draw with the “col” option; most likely you will have to define your own palette and number of contours to get a look you like. However, depending on how granulated are your data points (and thus bins), the resulting plot may not look as smooth as the figure.

[couet]

void graph2col() {
   TCanvas *c = new TCanvas("c","2 graphs with colors palette",700,500);

   // Create two graphs. The first one has a low frequency and the second one an high one
   const Int_t n = 200;
   TGraph *gr1 = new TGraph(n);
   TGraph *gr2 = new TGraph(n);

   Double_t min = 11111.;
   Double_t max = -11111.;

   Double_t x, y1, y2;
   for (Int_t i=0;i<n;i++) {
      x = i*0.02;
      y1 = 10*sin(x);
      gr1->SetPoint(i,x,y1);
      y2 = 10*sin(10*x);
      gr2->SetPoint(i,x,y2);
      if (y2 < min) min = y2;
      if (y2 > max) max = y2;
   }

   // Draw the first graph as a line
   gr1->Draw("AL");
   c->Update();

   // Retrieve the min value of the plot
   Double_t minp = gr1->GetHistogram()->GetMinimum();

   // Retrive the number of colors in the current palette
   Int_t nb = TColor::GetNumberOfColors();

   // Draw the second graph as colors below the first one.
   Int_t ic;
   for (Double_t x0 =0; x0<4.; x0 = x0+0.02) {
       ic = ((max - gr2->Eval(x0)) / (max-min))*nb;
       TLine *l = new TLine(x0 , minp, x0, gr1->Eval(x0));
       l->SetLineColor(TColor::GetColorPalette(ic));
       l->SetLineWidth(5);
       l->Draw();
   }

   // Draw again the first graph and redraw the axis
   gr1->SetLineWidth(5);
   gr1->SetLineColor(kRed);
   gr1->Draw("L");
   c->RedrawAxis();
}