'Returns vertical position of element identified by `locator`.
The position is returned in pixels off the top of the page,
as an integer. Fails if a matching element is not found.
See also `Get Horizontal Position`.'

@keyword
def get_vertical_position(self, locator):

    element = self.find_element(locator, required=False)
    if (element is None):
        raise AssertionError(("Could    not    determine    position    for    '%s'" % locator))
    return element.location['y']
