'Retrieve the most frequently seen items in a frequency table for each
interval in a series. (This is in contrast with ``get_most_frequent``,
which returns the most frequent items seen over the entire requested
range.)
Results are returned as a mapping, where the key is the key requested
and the value is a list of ``(timestamp, {item: score, ...})`` pairs
over the series. The maximum number of items returned for each interval
is the index capacity if no ``limit`` is provided.'

def get_most_frequent_series(self, model, keys, start, end=None, rollup=None, limit=None):

    raise NotImplementedError
