docs: def get_swell_url(self, swell_type):
          """Get swell arrow url."""
          if swell_type not in SWELL_TYPES:
              raise ValueError('Invalid swell type: {}'.format(swell_type))
          key = "swell_components_{}_direction".format(swell_type)
          swell_direction = self.f_d.get(key)
          if swell_direction is not None:
              rounded = int(5 * round(float(swell_direction)/5))
              return SWELL_ARROW_URL.format(rounded)