Reverse x axis, TGraph

ROOT
1

Hello,

to do a “simple” x:y plot of some data (read into a tree) I use TGraph. This works fine, but I want to reverse the x-axis. I can already reverse the direction, but I want as well that TGraph draws the plot from higher x-values to lower x-values. How does this work?

My code looks like this:

#include <TTree.h>
#include <TCanvas.h>
#include <TPad.h>
#include <TFile.h>
#include <TGraph.h>
#include <TGaxis.h>

void graph() {
// save as root file
    TFile* file = new TFile("data07.root", "RECREATE");
    file->cd();

// create Canvas
    gROOT->SetStyle("Plain");
    TCanvas *c1 = new TCanvas("c1","");

// read data in tree
    TTree *tree = new TTree("data","");
    tree->ReadFile("data.txt","TC:L");
    int size = tree->GetEntries();
    tree->SetEstimate(size);

// draw data as TGraph
    tree->Draw("TC:L");
    TGraph *g = new TGraph(size,tree->GetV1(),tree->GetV2());

    g->GetYaxis()->SetTitle("L");
    g->GetYaxis()->CenterTitle(1);
    g->GetXaxis()->SetTitle("T");
    g->GetXaxis()->CenterTitle(1);
    g->SetLineColor(1);        
    g->Draw("al");

// remove the current axis
    g->GetXaxis()->SetLabelOffset(999);
    g->GetXaxis()->SetTickLength(0);
// redraw the new axis
    gPad->Update();
    TGaxis *newaxis = new TGaxis(gPad->GetUxmax(),
                                gPad->GetUymin(),
                                gPad->GetUxmin(),
                                gPad->GetUymin(),
                                g->GetXaxis()->GetXmin(),
                                g->GetXaxis()->GetXmax(),
                                510,"-");
    newaxis->SetLabelOffset(-0.03);

    newaxis->Draw();

    c1->Write();
    c1->SaveAs("data07.eps");
}

Thanks a lot!
Boginja

2

This in on our to do list.
Right now you have to reverse the vectors yourself.

1 Like
3

Thanks!
And how can I do this? Do you have a small example how to reverse the vectors?

Cheers,
Boginja

4

You do a simple “for” loop and you copy the vectors in to new ones in the order you need them.
I do not think there is a need for an example.

5

Sorry to revive an old thread but did this item on your to-do list ever get done?

6

[quote=“couet”]You do a simple “for” loop and you copy the vectors in to new ones in the order you need them.
I do not think there is a need for an example.[/quote]

Also, in an X-Y graph this does not mirror the plot in the Y-axis as you are simply defining a set of points, the order in which this is done makes no difference.

The desired effect is obtained by multiplying all X values by -1, but then the axis labels are wrong. It is not impossible to implement in code, but it is not trivial. Has there been any progress on this?

7

The status is still the same.

8

Still the same with reversing axes ? :slight_smile:

9

This can be done using this example.
reversegraphXaxis.C (1.3 KB)

10

Would it be possible to have this as a native feature in the ROOT core (by adding a new method in TGraph and others for example)?

11

Done. Two new options are available in the ROOT master to draw TGraph with reverse axis.

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