def _check_fields(self, x, y):
		"""
		Check x and y fields parameters and initialize
		"""
		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
Check x and y fields parameters and initialize