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as per: http://apache-spark-user-list.1001560.n3.nabble.com/Using-TF-IDF-from-MLlib-td19429.html#a19528
Having the IDF.fit accept LabelPoints would be useful since, correct me if i'm wrong, there currently isn't a way of keeping track of which labels belong to which documents if one needs to apply a conventional tf-idf transformation on labelled text data.