New color schemes

(8 October 2024)

Choosing an appropriate color scheme is essential for making results easy to understand and interpret. Factors like colorblindness and converting colors to grayscale for publications can impact accessibility. Furthermore, results should be aesthetically pleasing. The following three color schemes, recommended by M. Petroff in arXiv:2107.02270v2 and available on GitHub under the MIT License, meet these criteria.

The following example demonstrates how to use the accessible color schemes (in this case, the one with six colors) to represent a THStack. It also shows that the grayscale version is an acceptable alternative.

void thstackcolorscheme()
{
   auto c1 = new TCanvas();
   auto hs = new THStack("hs","Stacked 1D histograms colored using 6-colors scheme");

   // Create six 1-d histograms  and add them in the stack
   auto h1st = new TH1F("h1st","A",100,-4,4);
   h1st->FillRandom("gaus",20000);
   h1st->SetFillColor(kP6Blue);
   hs->Add(h1st);

   auto h2st = new TH1F("h2st","B",100,-4,4);
   h2st->FillRandom("gaus",15000);
   h2st->SetFillColor(kP6Yellow);
   hs->Add(h2st);

   auto h3st = new TH1F("h3st","C",100,-4,4);
   h3st->FillRandom("gaus",10000);
   h3st->SetFillColor(kP6Red);
   hs->Add(h3st);

   auto h4st = new TH1F("h4st","D",100,-4,4);
   h4st->FillRandom("gaus",10000);
   h4st->SetFillColor(kP6Grape);
   hs->Add(h4st);

   auto h5st = new TH1F("h5st","E",100,-4,4);
   h5st->FillRandom("gaus",10000);
   h5st->SetFillColor(kP6Gray);
   hs->Add(h5st);

   auto h6st = new TH1F("h6st","F",100,-4,4);
   h6st->FillRandom("gaus",10000);
   h6st->SetFillColor(kP6Violet);
   hs->Add(h6st);

   // Draw the stack with colors.
   hs->Draw();
   TLegend *l = gPad->BuildLegend(.8,.55,1.,.9,"","F");
   l->SetLineWidth(0);
   l->SetFillStyle(0);

   // Draw the stack using gray scale.
   auto c2 = new TCanvas();
   c2->SetGrayscale();
   hs->Draw();
   l->Draw();
}

These new color schemes, now in the master version of ROOT, will be available in version 6.34.