Draw error band on already existed sub-canvas

[thegame]

Hi everyone,
I have a canvas saved as ‘abc.root’ that contains two sub-canvas. Currently, I want to add a band to the right side of the canvas, which is schematically represented as a yellow band over the entire x-axis range with a fixed value +/- error that will be manually entered. Please let me know if you have any recommendations. Thanks.

[couet]

It very much depends how are stored your data, but lets assume they are stored in two vectors. Then you can create a TGraph and draw it with option “F” (for filled).

[thegame]

@couet

It very much depends on how you store your data, but let’s assume they are stored in two vectors. Then you can create a TGraph and draw it with option “F” (for filled).

TGraphErrors* lnGraphErrors = new TGraphErrors(Xaxis.size(), &Xaxis[0], nullptr, nullptr, nullptr);

for (size_t i = 0; i < Xaxis.size(); ++i) {
    double RatioIn = Yaxis1[i];
    double RatioOut = Yaxis2[i];

    if (RatioIn != 0) {
        double lnRatio = std::log(RatioOut / RatioIn);
        double S = coeff * lnRatio;
        double SERR =  ((Yerror1[i] / RatioIn) * (Yerror1[i] / RatioIn) + (Yerror2[i] / RatioOut) * (Yerror2[i] / RatioOut)));
        lnGraphErrors->SetPoint(i, Xaxis[i], S);
        lnGraphErrors->SetPointError(i, 0.0, SERR); // x error is set to 0

      }
}
lnGraphErrors->SetMarkerColor(kRed);
lnGraphErrors->SetMarkerStyle(20);
canvas->cd(2);
gPad->SetGrid(1, 1);
lnGraphErrors->Draw("AP");

This is the relevant section on how canvas->cd(2) was generated initially, and then the canavs was saved as a root file (let’s say abc.root). Now I want to load the abc.root and access canvas->cd(2) from the root file to draw an error band of a fixed number +/- error (say, 850 +/- 4) on top of that, as shown above. I hope this makes the question clear. thanks again.

[couet]

You need something like that:

   auto f = new TFile("abc.root");
   TCanvas *c = (TCanvas *)f->Get("canvas");
   c->Draw();
   c->cd(2);
   ... build the TGraph g to draw the band
   g->SetFillColor(kYellow);
   g->Draw("F");

[thegame]

Thanks, @couet . I was not doing c->Draw() before setting it to c->cd(2), which is probably why it was not working. Also, the way I defined the TGraphErrors *g,

g->Draw(“3”);

works.

Thank you once again for your time.

[couet]

Yes is you have a TGraphErrors option 3 is the one to use.