# Problem with logarithmic color axis/colz with TNtuple

Hi,

I am trying to plot 3 sets of data using TNtuple with a logarithmic color/z-axis.
Actually in my case “SetLogz()” doesn’t work.
More precisely, it’s possible to make the z-axis logarithmic but it won’t be linked to the color code in the plot

Below one simple example :

``````#include <TStyle.h>
#include <TCanvas.h>
#include <TNtuple.h>
{
TNtuple* nt = new TNtuple("nt","","a:b:c");
TCanvas *Can = new TCanvas("Can","",200,10,600,400);

Double_t x[16] =  {1.00, 2.00, 6.00, 7.00, 3.00, 10.00, 0.20, 5.55,
20.50, 9.75, 1.50, 8.00, 5.00, 0.95, 3.85, 8.70};
Double_t y[16] = {1.00, 2.00, 6.00, 7.00, 3.00, 10.00, 0.20, 5.55,
20.50, 9.75, 1.50, 8.00, 5.00, 0.95, 3.85, 8.70};
Double_t z[16] = {0.001, 2.50, 600.00, 0.00001, 30.00, 10.001, 0.00020, 500000.55,
20.50, 9.75, 280000.50, 8.00, 5.00, 0.95, 3.85, 8.70};

Double_t x3, y3, z3;

for(Int_t h1 =0;h1<16;++h1)
{
x3 = x[h1];
y3 = y[h1];
z3 = z[h1]; nt->Fill(x3,y3,z3);
}
Can->SetTickx(1);
gStyle->SetOptStat(0);
gStyle->SetPalette(1,0);
nt->SetMarkerStyle(20);
nt->SetMarkerSize(1);
nt->Draw("b:a:c","c","colz");
}``````

Normally the point with z=280000.50 should be in red when z-axis is logarithmic, but it’s not the case.

Thanks

A brutal fix:

``````if (gPad->GetLogz()) nt->Draw("b:a:log10(c)*(c>0)", "", "colz");
else nt->Draw("b:a:c", "", "colz");
``````

Thanks for the answer but the problem still remains, z-axis is still disconnected to the color code in the plot.

For instance for the isolated point with z=20.5, if we take log10(20.5), approximately = 1.3, logarithmic scale shows us that z would be around 0.2 - 0.3.

Any other ideas?

Cheers.

This seems to be a bug in ROOT (the numbers on the “z” scale, i.e. on the color palette, are wrong in “Logz” mode) and it seems to be closely related to How plot only fit result of the TGraph?

OK, I posted a bug report.

Yes I know, no solution, or modify ROOT…