horizontalValuesAccumulatorByAddAllPartitionedPairedStats.yStats().mean());
    assertVerticalLinearTransformation(
        verticalValuesAccumulator.leastSquaresFit(), verticalValuesAccumulator.xStats().mean());
    assertVerticalLinearTransformation(
        verticalValuesAccumulatorByAddAllPartitionedPairedStats.leastSquaresFit(),

import static com.google.common.math.StatsTesting.assertLinearTransformationNaN;
import static com.google.common.math.StatsTesting.assertStatsApproxEqual;
import static com.google.common.math.StatsTesting.assertVerticalLinearTransformation;
import static com.google.common.math.StatsTesting.createPairedStatsOf;
import static com.google.common.truth.Truth.assertThat;
import static com.google.common.truth.Truth.assertWithMessage;

   * assertions that {@link LinearTransformation#slope} and {@link LinearTransformation#transform}
   * throw as expected.
   */
  static void assertVerticalLinearTransformation(LinearTransformation transformation, double x) {
    assertThat(transformation.isHorizontal()).isFalse();
    assertThat(transformation.isVertical()).isTrue();