tBlocks are stretching horizontally but not vertically!
  The reason is that a Vertical Panel never stretches its elements vertically.
  It always stacks its elements on top of each other with their natural heights.
  When a Vertical Panel is taller than the stack of its elements, there is extra space at the bottom.
  (Similarly, a Horizontal Panel never stretches its elements horizontally.)
</p>

<p>
  Instead of a Vertical Panel we should use a Table Panel.
  This requires assigning the <a>GraphObject.row