'Retrieve the total frequency of known items in a table over time.
The items requested should be passed as a mapping, where the key is the
metric key, and the value is a sequence of members to retrieve scores
for.
Results are returned as a mapping, where the key is the key requested
and the value is a mapping of ``{item: score, ...}`` containing the
total score of items over the interval.'

def get_frequency_totals(self, model, items, start, end=None, rollup=None):

    raise NotImplementedError
