'Override to set up the locators and formatters to use with the
scale.  This is only required if the scale requires custom
locators and formatters.  Writing custom locators and
formatters is rather outside the scope of this example, but
there are many helpful examples in ``ticker.py``.
In our case, the Mercator example uses a fixed locator from
-90 to 90 degrees and a custom formatter class to put convert
the radians to degrees and put a degree symbol after the
value::'

def set_default_locators_and_formatters(self, axis):

    class DegreeFormatter(Formatter, ):
        def __call__(self, x, pos=None):
            return (u'%d\xb0' % np.degrees(x))
    axis.set_major_locator(FixedLocator(np.radians(np.arange((-90), 90, 10))))
    axis.set_major_formatter(DegreeFormatter())
    axis.set_minor_formatter(DegreeFormatter())
