Checks if charts defaults are set

def _check_defaults(self, x_only=True):
		"""
		Checks if charts defaults are set
		"""
		if self.x is None:
			self.err(self._check_defaults,
					 "X field is not set: please specify a parameter")
			return
		if x_only is True:
			return
		if self.y is None:
			self.err(self._check_defaults,
					 "Y field is not set: please specify a parameter")
			return