def get_category_histogram( search_string = nil)
      raise "no search string provieded!" if( search_string === nil)

      fts = "fts=" + search_string.split().join( '+').to_s

      return call_api( __method__, fts)
    end
def get_category_histogram( search_string = nil)
      raise "no search string provieded!" if( search_string === nil)

      fts = "fts=" + search_string.split().join( '+').to_s

      return call_api( __method__, fts)
    end
This method returns a list of categories and product counts that describe the results
 of a given product query. The query is specified using the product query parameters.
 @param  [String]  search_string The string to be in the query.
 @return [String]  A list of Category objects. Each Category has an id, name, and count
   of the number of query results in that category.