def curve_data(self, target, clf_idx):
        """Return `ROCData' for the given target and classifier."""
        if (target, clf_idx) not in self._curve_data:
            data = ROCData.from_results(self.results, clf_idx, target)
            self._curve_data[target, clf_idx] = data

        return self._curve_data[target, clf_idx]