Return the next available `c:ser/c:order` value within the scope of
        this chart, the maximum order value found on existing series,
        incremented by one.

def next_order(self):
        """
        Return the next available `c:ser/c:order` value within the scope of
        this chart, the maximum order value found on existing series,
        incremented by one.
        """
        order_vals = [s.order.val for s in self.sers]
        if not order_vals:
            return 0
        return max(order_vals)+1