Can not update Canvas in a interactive mode of ROOT

Newbie
1

Dear experts

I want to draw a graph and change the title of that graph in a interactive mode, what I did is:

root -l something.root
tree->Draw("adc[24]:Iteration$>>g", "", "AL", 1, 0)
g->SetTitle("test")
c1->Modified()
c1->Update()

But the title was not changed after I typed such code. Why is that?

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ROOT Version: v6.26.02
Platform: Linux
Compiler: g++

2 
root [0] ntuple->Draw("px:py","","AL");
root [1] Graph->SetTitle("hello");
3

I got the error says:

use of undeclared identifier 'ntuple'
4

well, the tree I use to test is called “ntuple” …the one in hsimple.root. In your case it is “tree” …

5

Then I should did exactly what you did…but the canvas was not updated on the screen…

6

In my case with the latest root on Mac the canvas is created by the draw command then the title is displayed with the 2nd command.

7

I just tried again, when I use:

tree->Draw("py:px", "", "AL", 1, 0);
Graph->SetTitle("hello");
c1->Modified();

It worked (notice that if without c1->Modified() it didn’t work

But if I do this:

tree->Draw("py:px>>g", "", "AL", 1, 0);
g->SetTitle("hello");
c1->Modified();

It didn’t work.

SO I got 2 questions:

  1. In the second case, does >>g mean that copy the TGraph to a new one called “g”? So the original one been drawn is still called “Graph”
  2. Why I need c1->Modified() but you don’t…?
8

In that case “g” must be a 2D histogram not a TGraph see the doc

9

That’s true, but still I have the same two questions above… TGraph or TH2 should not affect the conditions, the first question will be like:

does >>g mean that copy the TH2 to a new one called “g”? So the original one been drawn is still called “Graph”

10

When you draw a TTree as a 2D scatter plot a TGraph called “Graph” is automatically created:
See:

root [0] ntuple->Draw("px:py")
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
root [1] gPad->ls()
Canvas Name=c1 Title=c1 Option=
 TCanvas fXlowNDC=0 fYlowNDC=0 fWNDC=1 fHNDC=1 Name= c1 Title= c1 Option=
  OBJ: TList	TList	Doubly linked list : 0
   TFrame  X1= -4.500000 Y1=-4.500000 X2=5.000000 Y2=5.000000
   OBJ: TH2F	htemp	px:py : 1 at: 0x7f8ca84c3c00
   OBJ: TPaveText	title  	X1= -0.413163 Y1=5.475000 X2=0.913163 Y2=6.128125
   OBJ: TGraph	Graph	Graph : 1 at: 0x600000434bb0 
root [2]
11

Ah, now I understood. >>g is just another object been created and nothing about the original Graph. Thank you!

12

Exactly, and the object after >> must be a histogram any way, not a TGraph.