@spec frequencies(dataframe(), facet_type(), keyword()) :: dataframe()

Compute count and frequency for a columns subset.

Options:

• filter - an Explorer.DataFrame filter function that excludes data in analytics
• counts - includes only facets with this counts (integer)
• sort - by :counts or :freq (frequencies over the years)

@spec rank_and_limit(
  dataframe(),
  keyword()
) :: dataframe()

Rank data frame by counts or (year) frequency, limits to nth rows.