code: def filter_data_columns(data):
      
      keys = set()
      for dt, d in data.items():
          for k in d:
              keys.add(k)
      return sorted([x for x in keys])
docs: Given a dict of data such as those in :py:class:`~.ProjectStats` attributes,
      made up of :py:class:`datetime.datetime` keys and values of dicts of column
      keys to counts, return a list of the distinct column keys in sorted order.
  
      :param data: data dict as returned by ProjectStats attributes
      :type data: dict
      :return: sorted list of distinct keys
      :rtype: ``list``