[tutorials] improve graphics/gtime.C tutorial

At each step draw newly created graph with reasonable content
Avoid writing value to vector out of bounds

tutorials/graphics/gtime.C

@@ -1,46 +1,49 @@
 /// \file
 /// \ingroup tutorial_graphics
 /// Example of a graph of data moving in time.
-/// Use the canvas "File/Quit" to exit from this example
+/// Use the canvas "File/Quit ROOT" to exit from this example
 ///
 /// \macro_code
 ///
 /// \author  Olivier Couet
 
 void gtime()
 {
-   auto c1 = (TCanvas*) gROOT->FindObject("c1");
+   auto c1 = (TCanvas *) gROOT->FindObject("c1");
    if (c1) delete c1;
 
    c1 = new TCanvas("c1");
    const Int_t ng = 100;
    const Int_t kNMAX = 10000;
    std::vector<Double_t> X(kNMAX), Y(kNMAX);
    Int_t cursor = kNMAX;
-   TGraph *g = new TGraph(ng);
-   g->SetMarkerStyle(21);
-   g->SetMarkerColor(kBlue);
-   Double_t x = 0;
+   Double_t x = 0, stepx = 0.1;
 
-   while (1) {
-      c1->Clear();
-      if (cursor > kNMAX-ng) {
+   while (!gSystem->ProcessEvents()) {
+      if (cursor + ng >= kNMAX) {
+         cursor = 0;
          for (Int_t i = 0; i < ng; i++) {
             X[i] = x;
-            Y[i] = sin(x);
-            x += 0.1;
+            Y[i] = TMath::Sin(x);
+            x += stepx;
          }
-         g->Draw("alp");
-         cursor = 0;
       } else {
-         x += 0.1;
          X[cursor+ng] = x;
          Y[cursor+ng] = TMath::Sin(x);
+         x += stepx;
          cursor++;
-         g->DrawGraph(ng, X.data()+cursor, Y.data()+cursor, "alp");
       }
+
+      TGraph *g = new TGraph(ng, X.data()+cursor, Y.data()+cursor);
+      g->SetMarkerStyle(21);
+      g->SetMarkerColor(kBlue);
+      g->SetLineColor(kGreen);
+      g->SetBit(kCanDelete); // let canvas delete graph when call TCanvas::Clear()
+
+      c1->Clear();
+      g->Draw("alp");
       c1->Update();
-      if (gSystem->ProcessEvents()) break;
+
       gSystem->Sleep(10);
    }
 }