code: def category_axis(self):
          
          catAx_lst = self._chartSpace.catAx_lst
          if catAx_lst:
              return CategoryAxis(catAx_lst[0])
  
          dateAx_lst = self._chartSpace.dateAx_lst
          if dateAx_lst:
              return DateAxis(dateAx_lst[0])
  
          valAx_lst = self._chartSpace.valAx_lst
          if valAx_lst:
              return ValueAxis(valAx_lst[0])
  
          raise ValueError('chart has no category axis')
docs: The category axis of this chart. In the case of an XY or Bubble
          chart, this is the X axis. Raises |ValueError| if no category
          axis is defined (as is the case for a pie chart, for example).