def _update_perf_line(self):
        if self._perf_line is None:
            return

        self._perf_line.setVisible(self.display_perf_line)
        if self.display_perf_line:
            m = roc_iso_performance_slope(
                self.fp_cost, self.fn_cost, self.target_prior / 100.0)

            hull = self._rocch
            ind = roc_iso_performance_line(m, hull)
            angle = numpy.arctan2(m, 1)  # in radians
            self._perf_line.setAngle(angle * 180 / numpy.pi)
            self._perf_line.setPos((hull.fpr[ind[0]], hull.tpr[ind[0]]))