Setting Log Scale on just y-axis of a TH2F graph

ROOT
,
7

Indeed, sorry for the confusion. If what you want is to change the lowest exponent shown on the axis, try SetLimits:

 {
  auto c1    = new TCanvas("c1","c1",1500,500);
  c1->Divide(3,1);

  auto hcol1 = new TH2F("hcol1","Original",40,-4,4,40,0,700);
  float px, py;
  for (Int_t i = 0; i < 50000; i++) {
    gRandom->Rannor(px,py);
    hcol1->Fill(px,100*py);
  }

  c1->cd(1);
  hcol1->DrawCopy("COLZ");

  c1->cd(2);
  gPad->SetLogy(1);
  hcol1->SetTitle("Logy");
  hcol1->DrawCopy("COLZ");

  c1->cd(3);
  gPad->SetLogy(1);
  hcol1->SetTitle("Logy + SetLimits");
  hcol1->GetYaxis()->SetLimits(1e-1,500);
  hcol1->DrawCopy("COLZ");
}

c1
c11498×476 10.2 KB

Edit: Note, by the way, that SetLimits may not be the best option, as it can alter your histogram (see this).