Axes Lables and Title on a Scatter Plot

ROOT
1

Hello-

I’m very new to Root, so my apologies for asking such a basic question. However, I’ve spent a couple of hours trying to figure this out and I haven’t really gotten anywhere.

I’ve been having trouble getting axes labels and a title on a scatter plot. I have spent quite a bit of time with the tutorials and examples, but few seem to actually have titles and labels, and the ones that do seem to include them manually.

I’ve included a minimum example showing some of the things that I’ve tried. The first canvas appears as I expect it to, but the same technique doesn’t seem to work on the second one.

Any help/pointers/suggestions are welcome.
Thanks in advance,
–Chad

void minEx(){
	TFile * f = new TFile("temp.root", "RECREATE");
	TTree * dat = new TTree("data", "Tree containing x,y data");
	Float_t x, y;
	dat->Branch("x", &x, "x/F");
	dat->Branch("y", &y, "y/F");

	for (Int_t i=0; i < 10000; i++){
		gRandom->Rannor(x, y);
		dat->Fill();
	}

	TCanvas * c1 = new TCanvas("TheHistogramCanvas");
	dat->Draw("y:x >> xyhist", "", "LEGO1");
	TH2F * hist = f->Get("xyhist");
	// This works
	hist->SetTitle("This is the Histogram Title");
	hist->GetXaxis()->SetTitle("This is the histogram x-axis");
	hist->GetXaxis()->SetTitleOffset(2);
	hist->GetYaxis()->SetTitle("This is the histogram y-axis");
	hist->GetYaxis()->SetTitleOffset(2);

	TCanvas * c2 = new TCanvas("TheScatterCanvas");
	dat->Draw("y:x >> xyscatter");
	TH2F * scatter = f->Get("xyscatter");
	// This does not work
	scatter->SetTitle("This Does Not Set the Scatter Title");
	scatter->GetXaxis()->SetTitle("This does not set the scatter x-axis");
	scatter->GetYaxis()->SetTitle("This does not set the scatter y-axis");

	// This does not work
	TGraph * g = gPad->FindObject("Graph");
	g->SetTitle("This Does Not Set the Scatter Title");
	g->GetXaxis()->SetTitle("This does not set the scatter x-axis");
	g->GetYaxis()->SetTitle("This does not set the scatter y-axis");

	// This will allow me to add lines but not to change or remove the
	// original title line
	TPaveText * tp = gPad->FindObject("title");
	// shows the text that is currently shown as the title
	TText * line = tp->GetLine(0);
	line->Print();
	line->SetText(0, 0, "This Does Not Set the Scatter Title");
	// This adds a second line to the title
	tp->AddText("Second Scatter Title Line");
}
2 
root [0] ntuple->Draw("px:py");
root [1] htemp->SetTitle("Main Title ; X Title ; Y Title")
3

Thank you, using htemp in that way seems to work.

Can you explain, or point me to an explanation, why this works using htemp but does not work the way I had set it up? I don’t understand the difference between using the htemp pointer and getting the histogram pointer via the file structure. I’d like to understand this so that I might be able to better solve my own problems in the future.

Thank you very much,
–Chad

4

htemp is the frame histogram containing the result of your query. htemp is created automatically when you do not specify any histogram. You can also use you own histogram a follow. It works also:

root [0] TH2D *h2 = new TH2D("h2","h2",40,-10,10,40,-10,10)
root [1] ntuple->Draw("px:py >> h2");
root [2] h2->SetTitle("Main Title ; X Title ; Y Title")