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.

def add_category(self, label):
        """
        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.
        """
        category = Category(label, self)
        self._categories.append(category)
        return category