const CImg<T>& _display_graph(CImgDisplay &disp, const char *const title=0,
                                  const unsigned int plot_type=1, const unsigned int vertex_type=1,
                                  const char *const labelx=0, const double xmin=0, const double xmax=0,
                                  const char *const labely=0, const double ymin=0, const double ymax=0,
                                  const bool exit_on_anykey=false) const {
      if (is_empty())
        throw CImgInstanceException(_cimg_instance
                                    "display_graph(): Empty instance.",
                                    cimg_instance);
      if (!disp) disp.assign(cimg_fitscreen(CImgDisplay::screen_width()/2,CImgDisplay::screen_height()/2,1),0,0).
                   set_title(title?"%s":"CImg<%s>",title?title:pixel_type());
      const ulongT siz = (ulongT)_width*_height*_depth, siz1 = std::max((ulongT)1,siz - 1);
      const unsigned int old_normalization = disp.normalization();
      disp.show().flush()._normalization = 0;

      double y0 = ymin, y1 = ymax, nxmin = xmin, nxmax = xmax;
      if (nxmin==nxmax) { nxmin = 0; nxmax = siz1; }
      int x0 = 0, x1 = width()*height()*depth() - 1, key = 0;

      for (bool reset_view = true; !key && !disp.is_closed(); ) {
        if (reset_view) { x0 = 0; x1 = width()*height()*depth() - 1; y0 = ymin; y1 = ymax; reset_view = false; }
        CImg<T> zoom(x1 - x0 + 1,1,1,spectrum());
        cimg_forC(*this,c) zoom.get_shared_channel(c) = CImg<T>(data(x0,0,0,c),x1 - x0 + 1,1,1,1,true);
        if (y0==y1) { y0 = zoom.min_max(y1); const double dy = y1 - y0; y0-=dy/20; y1+=dy/20; }
        if (y0==y1) { --y0; ++y1; }

        const CImg<intT> selection = zoom.get_select_graph(disp,plot_type,vertex_type,
                                                           labelx,
                                                           nxmin + x0*(nxmax - nxmin)/siz1,
                                                           nxmin + x1*(nxmax - nxmin)/siz1,
                                                           labely,y0,y1,true);
        const int mouse_x = disp.mouse_x(), mouse_y = disp.mouse_y();
        if (selection[0]>=0) {
          if (selection[2]<0) reset_view = true;
          else {
            x1 = x0 + selection[2]; x0+=selection[0];
            if (selection[1]>=0 && selection[3]>=0) {
              y0 = y1 - selection[3]*(y1 - y0)/(disp.height() - 32);
              y1-=selection[1]*(y1 - y0)/(disp.height() - 32);
            }
          }
        } else {
          bool go_in = false, go_out = false, go_left = false, go_right = false, go_up = false, go_down = false;
          switch (key = (int)disp.key()) {
          case cimg::keyHOME : reset_view = true; key = 0; disp.set_key(); break;
          case cimg::keyPADADD : go_in = true; go_out = false; key = 0; disp.set_key(); break;
          case cimg::keyPADSUB : go_out = true; go_in = false; key = 0; disp.set_key(); break;
          case cimg::keyARROWLEFT : case cimg::keyPAD4 : go_left = true; go_right = false; key = 0; disp.set_key();
            break;
          case cimg::keyARROWRIGHT : case cimg::keyPAD6 : go_right = true; go_left = false; key = 0; disp.set_key();
            break;
          case cimg::keyARROWUP : case cimg::keyPAD8 : go_up = true; go_down = false; key = 0; disp.set_key(); break;
          case cimg::keyARROWDOWN : case cimg::keyPAD2 : go_down = true; go_up = false; key = 0; disp.set_key(); break;
          case cimg::keyPAD7 : go_left = true; go_up = true; key = 0; disp.set_key(); break;
          case cimg::keyPAD9 : go_right = true; go_up = true; key = 0; disp.set_key(); break;
          case cimg::keyPAD1 : go_left = true; go_down = true; key = 0; disp.set_key(); break;
          case cimg::keyPAD3 : go_right = true; go_down = true; key = 0; disp.set_key(); break;
          }
          if (disp.wheel()) {
            if (disp.is_keyCTRLLEFT() || disp.is_keyCTRLRIGHT()) go_up = !(go_down = disp.wheel()<0);
            else if (disp.is_keySHIFTLEFT() || disp.is_keySHIFTRIGHT()) go_left = !(go_right = disp.wheel()>0);
            else go_out = !(go_in = disp.wheel()>0);
            key = 0;
          }

          if (go_in) {
            const int
              xsiz = x1 - x0,
              mx = (mouse_x - 16)*xsiz/(disp.width() - 32),
              cx = x0 + cimg::cut(mx,0,xsiz);
            if (x1 - x0>4) {
              x0 = cx - 7*(cx - x0)/8; x1 = cx + 7*(x1 - cx)/8;
              if (disp.is_keyCTRLLEFT() || disp.is_keyCTRLRIGHT()) {
                const double
                  ysiz = y1 - y0,
                  my = (mouse_y - 16)*ysiz/(disp.height() - 32),
                  cy = y1 - cimg::cut(my,0.,ysiz);
                y0 = cy - 7*(cy - y0)/8; y1 = cy + 7*(y1 - cy)/8;
              } else y0 = y1 = 0;
            }
          }
          if (go_out) {
            if (x0>0 || x1<(int)siz1) {
              const int delta_x = (x1 - x0)/8, ndelta_x = delta_x?delta_x:(siz>1);
              const double ndelta_y = (y1 - y0)/8;
              x0-=ndelta_x; x1+=ndelta_x;
              y0-=ndelta_y; y1+=ndelta_y;
              if (x0<0) { x1-=x0; x0 = 0; if (x1>=(int)siz) x1 = (int)siz1; }
              if (x1>=(int)siz) { x0-=(x1 - siz1); x1 = (int)siz1; if (x0<0) x0 = 0; }
            }
          }
          if (go_left) {
            const int delta = (x1 - x0)/5, ndelta = delta?delta:1;
            if (x0 - ndelta>=0) { x0-=ndelta; x1-=ndelta; }
            else { x1-=x0; x0 = 0; }
            go_left = false;
          }
          if (go_right) {
            const int delta = (x1 - x0)/5, ndelta = delta?delta:1;
            if (x1 + ndelta<(int)siz) { x0+=ndelta; x1+=ndelta; }
            else { x0+=(siz1 - x1); x1 = (int)siz1; }
            go_right = false;
          }
          if (go_up) {
            const double delta = (y1 - y0)/10, ndelta = delta?delta:1;
            y0+=ndelta; y1+=ndelta;
            go_up = false;
          }
          if (go_down) {
            const double delta = (y1 - y0)/10, ndelta = delta?delta:1;
            y0-=ndelta; y1-=ndelta;
            go_down = false;
          }
        }
        if (!exit_on_anykey && key && key!=(int)cimg::keyESC &&
            (key!=(int)cimg::keyW || (!disp.is_keyCTRLLEFT() && !disp.is_keyCTRLRIGHT()))) {
          disp.set_key(key,false);
          key = 0;
        }
      }
      disp._normalization = old_normalization;
      return *this;
    }