code: def add_category(self, label):
          
          category = Category(label, self)
          self._categories.append(category)
          return category
docs: Return a newly created |data.Category| object having *label* and
          appended to the end of this category sequence. *label* can be
          a string, a number, a datetime.date, or datetime.datetime object. All
          category labels in a chart must be the same type. All category labels
          in a chart having multi-level categories must be strings.
  
          Creating a chart from chart data having date categories will cause
          the chart to have a |DateAxis| for its category axis.