def filter_data_columns(data):
keys = set()<EOL>for dt, d in data.items():<EOL><INDENT>for k in d:<EOL><INDENT>keys.add(k)<EOL><DEDENT><DEDENT>return sorted([x for x in keys])<EOL>
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``