void GraphDoubleString(TGraph *gr, std::string c[]) {

   gr->Draw("ALP");
   gr->GetXaxis()->SetLabelSize(0);
   gr->GetXaxis()->SetTickLength(0);

   Int_t i,n;
   Double_t x,y;
   TLatex *t;
   TLine *tick;
   TLine *grid;
         printf("%s\n",c[0].c_str());

   double ymin = gr->GetHistogram()->GetMinimum();
   double ymax = gr->GetHistogram()->GetMaximum();
   double dy   = (ymax-ymin);
   n = gr->GetN();

   for (i=0; i<n; i++) {
      gr->GetPoint(i,x,y);
      t = new TLatex(x, ymin-0.03*dy, c[i].c_str());
      t->SetTextSize(0.025);
      t->SetTextFont(42);
      t->SetTextAlign(21);
      t->Draw();
      tick = new TLine(x,ymin,x,ymin+0.03*dy);
      tick->Draw();
      grid = new TLine(x,ymin,x,y);
      grid->SetLineStyle(3);
      grid->Draw();
   }
}

void plot() {
   Double_t x[3] = {1,2,3};  
   Double_t y[3] = {1,5,2}; 
   std::string colour[3] = { "Blue", "Red", "Green"}; 

   auto gr = new TGraph(3,x,y);
   gr->SetMarkerStyle(20);
   GraphDoubleString(gr,colour);
}