def category_axis(self):
        """
        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).
        """
        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')