code: def _check_fields(self, x, y):
  		
  		if x is None:
  			if self.x is None:
  				self.err(
  					self._check_fields,
  					"X field is not set: please specify a parameter")
  				return
  			x = self.x
  		if y is None:
  			if self.y is None:
  				self.err(
  					self._check_fields,
  					"Y field is not set: please specify a parameter")
  				return
  			y = self.y
  		return x, y
docs: Check x and y fields parameters and initialize