code: def get_coi(self, params_dict):
          
          lat = str(params_dict['lat'])
          lon = str(params_dict['lon'])
          start = params_dict['start']
          interval = params_dict['interval']
  
          # build request URL
          if start is None:
              timeref = 'current'
          else:
              if interval is None:
                  timeref = self._trim_to(timeformatutils.to_date(start), 'year')
              else:
                  timeref = self._trim_to(timeformatutils.to_date(start), interval)
  
          fixed_url = '%s/%s,%s/%s.json' % (CO_INDEX_URL, lat, lon, timeref)
          uri = http_client.HttpClient.to_url(fixed_url, self._API_key, None)
          _, json_data = self._client.cacheable_get_json(uri)
          return json_data
docs: Invokes the CO Index endpoint
  
          :param params_dict: dict of parameters
          :returns: a string containing raw JSON data
          :raises: *ValueError*, *APICallError*