Use a grid to manage all compositions

In turn we use a single LayoutTree class.

Author: has2k1

plotnine/_mpl/layout_manager/_grid.py

@@ -79,15 +79,15 @@ def __setitem__(
 
     def iter_rows(self) -> Iterator[list[T | None]]:
         """
-        Yield rows
+        Row by row
         """
         n = self._grid.shape[0]
         for row in range(n):
             yield self[row, :]
 
     def iter_cols(self) -> Iterator[list[T | None]]:
         """
-        Yield cols
+        Column by column
         """
         n = self._grid.shape[1]
         for col in range(n):