Helper function for panning and zooming over a set of marks.

    Creates and returns a panzoom interaction with the 'x' and 'y' dimension
    scales of the specified marks.

def panzoom(marks):
    """Helper function for panning and zooming over a set of marks.

    Creates and returns a panzoom interaction with the 'x' and 'y' dimension
    scales of the specified marks.
    """
    return PanZoom(scales={
            'x': sum([mark._get_dimension_scales('x', preserve_domain=True) for mark in marks], []),
            'y': sum([mark._get_dimension_scales('y', preserve_domain=True) for mark in marks], [])
    })