docs: def display_unit(self):
          """
          Display unit of value.
  
          :type: ``str``
          """
          if self._display_unit:
              return self._display_unit
          elif self._Q:
              config = Configuration.display.unit_systems
              default_system = Configuration.unit_system
              units = config.systems[default_system]
  
              self._display_unit = units.get(self._type, self._unit)
  
              if self._type == "temperature":
                  from_unit = "deg" + self._unit.upper()
                  to_unit = "deg" + self._display_unit.upper()
              else:
                  from_unit = self._unit
                  to_unit = self._display_unit
  
              #print("dv", from_unit, to_unit)
              self._q_unit = self._Q("1 " + from_unit)
              self._q_display = self._Q("1 " + to_unit)
              
              
          return self._display_unit