'Override this method to provide a transformation for the
x-axis tick labels.
Returns a tuple of the form (transform, valign, halign)'

def get_xaxis_transform(self, which=u'grid'):

    if (which not in [u'tick1', u'tick2', u'grid']):
        msg = u"'which'    must    be    on    of    [    'tick1'    |    'tick2'    |    'grid'    ]"
        raise ValueError(msg)
    return self._xaxis_transform
