'Set the number of degrees between each longitude grid.
This is an example method that is specific to this projection
class -- it provides a more convenient interface to set the
ticking than set_xticks would.'

def set_longitude_grid(self, degrees):

    grid = np.arange(((-180) + degrees), 180, degrees)
    self.xaxis.set_major_locator(FixedLocator(np.deg2rad(grid)))
    self.xaxis.set_major_formatter(self.ThetaFormatter(degrees))
