'Generate multiple ``VBar`` renderers for levels stacked bottom
to top.
Args:
stackers (seq[str]) : a list of data source field names to stack
successively for ``left`` and ``right`` bar coordinates.
Any additional keyword arguments are passed to each call to ``vbar``.
If a keyword value is a list or tuple, then each call will get one
value from the sequence.
Examples:
Assuming a ``ColumnDataSource`` named ``source`` with columns
*2106* and *2017*, then the following call to ``vbar_stack`` will
will create two ``VBar`` renderers that stack:
.. code-block:: python
p.vbar_stack([\'2016\', \'2017\'], x=10, width=0.9, color=[\'blue\', \'red\'], source=source)
This is equivalent to the following two separate calls:
.. code-block:: python
p.vbar(bottom=stack(),       top=stack(\'2016\'),         x=10, width=0.9, color=\'blue\', source=source)
p.vbar(bottom=stack(\'2016\'), top=stack(\'2016\', \'2017\'), x=10, width=0.9, color=\'red\', source=source)'

def vbar_stack(self, stackers, **kw):

    for kw in _stack(stackers, 'bottom', 'top', **kw):
        self.vbar(**kw)
