Rescaling axis

ROOT
1

Hi,

I am wondering how to rescale the X axis of an existing histogram. I tried the following session, and I got behaviours that I don’t understand :

root [27] c = new TCanvas()
root [28] h = new TH1F("test", "test", 20, 0, 20)
(class TH1F*)0x94f9678
root [29] h->Draw()
root [30] h->Fill(4)
(Int_t)(5)
root [31] h->Fill(23)
(Int_t)(-1)
root [32] h->GetXaxis()->Set(30, 0, 30)
root [33] h->RebinAxis(30, h->GetXaxis())
root [34] h->Fill(27)
(Int_t)(28)
root [35] h->Fill(10)
(Int_t)(11)
root [36] h->Fill(15)
(Int_t)(16)

[28] I create an histo with 20 bins and between 0 and 20
[31] Here I fill with a value which is outside the bounds. However, in the upper right legend it is written : Entries 2; Mean 4. This is not consistent, one of those two values is wrong.
[32] Change the axis to 30 bins between 0 and 30. The legend is still wrong. I have a strange bar appearing between 20 and 30. See the first snapshot.
[33] I try to rebin the axis, thinking that could help -> still the same
[34-36] I add a few points, the legend is still not consistent, and the data above 20 are not appearing.

I don’t understand what I did wrong and if this legend’s problem is “normal”.

Moreover, does some kind of auto-scaling histogram exist ? an histo able to automatically change the axes if data out of bounds are added ?

Thanks in advance !

Barth

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rootSnap1.png873×622 7.02 KB

2

Not sure what you want to do. Did you try h->Scale(somefactor); ?

Rene

3

Hi,

Our problem is that we have a histogram but we don’t know what data we will have to put in it. As a consequence we create it with a default X axis range and ask the user to manually change it to whatever is relevant for the displayed data.

However this is not really smart and I was wondering if there is any way to change the range of the axis, either automatically (the histogram is aware that the biggest value is x and adapt the axis accordingly) or manually (it is what I tried in the example that I put in the previous message, with behaviours which seem to be bugs ? ).

The scale method is not relevant in our case, because the data should stay as they are, it is the axis range which should change.

I hope that I have been more understandable :slight_smile:

Barth

4

Why don’t you use the histogramming automatic binning??
TH1F *h = new TH1F(“h”,“test”,100,0,0); //upper limit ,+ lower limit
h->Fill(some variable); //your loop filling the histo

as simple as that

Rene

5

Hi,

Thanks for your reply. I tried the automatic binning and it works for the first value, but not for the next ones. Here is an example :

root [0] h = new TH1F("h","test",100,0,0);
root [1] h->Fill(5)
(Int_t)(-2)
root [2] c = new TCanvas
(class TCanvas*)0x93bc158
root [3] h->Draw()
root [4] h->Fill(10)
(Int_t)(-2)

The second value (10) is not displayed because the range of the axis goes from ~4 to ~6. I would like it to (automatically?) change it in order to display all the values. How can I do that ?

Moreover, I am still uncomfortable with the fact that the legend says : Entries 2, Mean 5. For me, this is wrong. As we put the values 5 and 10 (not displayed) we should have either 1 entry and Mean 5 or 2 entries and mean 7.5.

Thanks for your help !

Barth

6

The automatic rebinning algorithm is reset once you make an operation on your histogram, including drawing the histogram. You can activate the algorithm (after Draw) again by calling:

h->SetBit(TH1::kCanRebin);

[quote]Moreover, I am still uncomfortable with the fact that the legend says : Entries 2, Mean 5. For me, this is wrong. As we put the values 5 and 10 (not displayed) we should have either 1 entry and Mean 5 or 2 entries and mean 7.5.
[/quote]Underflows/overflows are not counted (by default) when computing the mean/rms.
see TH1::StatOverflows

Rene

7

Thanks a lot, it is exactly what I was looking for.

And thanks for the explanation about the overflow.

Barth