[graf2d] allow zero outer margins when dividing canvas

When nx or ny is negative, the pads is divided according to abs(nx) or abs(ny), but the outer margins is set to 0. The inner margins are equal to xmargin and ymargin, respectively.

Author: rlalik

graf2d/gpad/src/TPad.cxx

@@ -1254,6 +1254,9 @@ Int_t TPad::DistancetoPrimitive(Int_t px, Int_t py)
 /// __Note3:__  in case xmargin < 0 or ymargin < 0, there is no space
 ///             between pads. The current pad margins are recomputed to
 ///             optimize the layout.
+///
+/// __Note4:__  in case nx < 0 or ny < 0, there is no outer x or y margin, but
+///             the inner margins are equal to xmargin and ymargin, respectively.
 
 void TPad::Divide(Int_t nx, Int_t ny, Float_t xmargin, Float_t ymargin, Int_t color)
 {
@@ -1269,30 +1272,44 @@ void TPad::Divide(Int_t nx, Int_t ny, Float_t xmargin, Float_t ymargin, Int_t co
    TContext ctxt(kTRUE);
 
    cd();
-   if (nx <= 0) nx = 1;
-   if (ny <= 0) ny = 1;
-   Int_t ix, iy;
-   Double_t x1, y1, x2, y2, dx, dy;
-   TPad *pad;
+   if (nx == 0)
+      nx = 1;
+   if (ny == 0)
+      ny = 1;
+
+   // Outer margins, equal to half of the inner margin. Set to zero if nx or ny are negative.
+   Float_t xmargin_outer = xmargin;
+   Float_t ymargin_outer = ymargin;
+
+   if (nx < 0) {
+      xmargin_outer = 0.0;
+      nx = -nx;
+   }
+
+   if (ny < 0) {
+      ymargin_outer = 0.0;
+      ny = -ny;
+   }
+
    TString name, title;
    Int_t n = 0;
    if (color == 0) color = GetFillColor();
    if (xmargin >= 0 && ymargin >= 0) {
       //general case
-      dy = 1/Double_t(ny);
-      dx = 1/Double_t(nx);
-      for (iy=0;iy<ny;iy++) {
-         y2 = 1 - iy*dy - ymargin;
-         y1 = y2 - dy + 2*ymargin;
+      auto dx = (1 - 2 * xmargin * (nx - 1) - 2 * xmargin_outer) / nx; // width of a subpad
+      auto dy = (1 - 2 * ymargin * (ny - 1) - 2 * ymargin_outer) / ny; // height of a subpad
+      for (auto iy = 0; iy < ny; iy++) {
+         auto y2 = 1 - iy * dy - 2 * ymargin * iy - ymargin_outer;
+         auto y1 = y2 - dy;
          if (y1 < 0) y1 = 0;
          if (y1 > y2) continue;
-         for (ix=0;ix<nx;ix++) {
-            x1 = ix*dx + xmargin;
-            x2 = x1 +dx -2*xmargin;
+         for (auto ix = 0; ix < nx; ix++) {
+            auto x1 = ix * dx + 2 * xmargin * ix + xmargin_outer;
+            auto x2 = x1 + dx;
             if (x1 > x2) continue;
             n++;
             name.Form("%s_%d", GetName(), n);
-            pad = new TPad(name.Data(), name.Data(), x1, y1, x2, y2, color);
+            auto pad = new TPad(name.Data(), name.Data(), x1, y1, x2, y2, color);
             pad->SetNumber(n);
             pad->Draw();
          }
@@ -1311,23 +1328,23 @@ void TPad::Divide(Int_t nx, Int_t ny, Float_t xmargin, Float_t ymargin, Int_t co
       SetRightMargin(xr);
       SetBottomMargin(yb);
       SetTopMargin(yt);
-      dx = (1-xl-xr)/nx;
-      dy = (1-yb-yt)/ny;
+      auto dx = (1 - xl - xr) / nx;
+      auto dy = (1 - yb - yt) / ny;
       Int_t number = 0;
       for (Int_t i=0;i<nx;i++) {
-         x1 = i*dx+xl;
-         x2 = x1 + dx;
+         auto x1 = i * dx + xl;
+         auto x2 = x1 + dx;
          if (i == 0) x1 = 0;
          if (i == nx-1) x2 = 1-xr;
          for (Int_t j=0;j<ny;j++) {
             number = j*nx + i +1;
-            y2 = 1 -j*dy -yt;
-            y1 = y2 - dy;
+            auto y2 = 1 - j * dy - yt;
+            auto y1 = y2 - dy;
             if (j == 0)    y2 = 1-yt;
             if (j == ny-1) y1 = 0;
             name.Form("%s_%d", GetName(), number);
             title.Form("%s_%d", GetTitle(), number);
-            pad = new TPad(name.Data(), title.Data(), x1, y1, x2, y2, color);
+            auto pad = new TPad(name.Data(), title.Data(), x1, y1, x2, y2, color);
             pad->SetNumber(number);
             pad->SetBorderMode(0);
             if (i == 0)    pad->SetLeftMargin(xl*nx);