code: def _set_axis(self, param, unit):
          
          axisValues = []
          for astroObject in self.objectList:
              try:
                  value = eval('astroObject.{0}'.format(param))
              except ac.HierarchyError:  # ie trying to call planet.star and one planet is a lone ranger
                  value = np.nan
  
              if unit is None:  # no unit to rescale (a aq.unitless quanitity would otherwise fail with ValueError)
                  axisValues.append(value)
              else:
                  try:
                      axisValues.append(value.rescale(unit))
                  except AttributeError:  # either nan or unitless
                      axisValues.append(value)
  
          return axisValues
docs: this should take a variable or a function and turn it into a list by evaluating on each planet