Issue 1:
Per conversation in issue #2507, IsotonicRegression appears to have regressed due to commit a9ea55f.

This IPython notebook demonstrates the failure on HEAD.

I tested the following two commits with the notebook:

d255866: no difference, SUCCESS

a9ea55f: difference, FAILURE

In other words, I think we can blame the switch for interp1d from "linear" to "slinear"; first thought is that 1-d spline "slinear" matrix formulation is ill-posed for x-ties, whereas the piecewise "linear" implementation is unaffected?

Small additional note: confirmed failure with test case where x-values are all non-zero, e.g., [1, 1, 2, 3, 4] instead of [0, 0, 1, 2, 3], so x=0 isn't part of the cause.

Issue 2:
Since we're now using upstream numpydoc, it should be fairly easy to fix the remaining errors in building the docs:

/home/ubuntu/scikit-learn/doc/modules/cross_validation.rst:287: WARNING: undefined label: time_series_cv (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/cross_validation.rst:287: WARNING: undefined label: group_cv (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/covariance/outlier_detection.py:docstring of sklearn.covariance.EllipticEnvelope.correct_covariance:4: WARNING: citation not found: Rouseeuw1984
/home/ubuntu/scikit-learn/sklearn/covariance/outlier_detection.py:docstring of sklearn.covariance.EllipticEnvelope.reweight_covariance:4: WARNING: citation not found: Rouseeuw1984
/home/ubuntu/scikit-learn/sklearn/covariance/robust_covariance.py:docstring of sklearn.covariance.MinCovDet.correct_covariance:4: WARNING: citation not found: Rouseeuw1984
/home/ubuntu/scikit-learn/sklearn/covariance/robust_covariance.py:docstring of sklearn.covariance.MinCovDet.reweight_covariance:4: WARNING: citation not found: Rouseeuw1984
/home/ubuntu/scikit-learn/doc/modules/generated/sklearn.linear_model.RandomizedLasso.rst:17: WARNING: undefined label: randomized_l1 (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/linear_model/randomized_l1.py:docstring of sklearn.linear_model.RandomizedLasso:25: WARNING: undefined label: randomized_l1 (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/linear_model/randomized_l1.py:docstring of sklearn.linear_model.RandomizedLasso:153: WARNING: undefined label: sphx_glr_auto_examples_linear_model_plot_sparse_recovery.py (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/generated/sklearn.linear_model.RandomizedLogisticRegression.rst:17: WARNING: undefined label: randomized_l1 (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/linear_model/randomized_l1.py:docstring of sklearn.linear_model.RandomizedLogisticRegression:27: WARNING: undefined label: randomized_l1 (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/linear_model/randomized_l1.py:docstring of sklearn.linear_model.RandomizedLogisticRegression:138: WARNING: undefined label: sphx_glr_auto_examples_linear_model_plot_sparse_recovery.py (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/generated/sklearn.linear_model.lasso_stability_path.rst:11: WARNING: undefined label: randomized_l1 (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/utils/deprecation.py:docstring of sklearn.linear_model.lasso_stability_path:67: WARNING: undefined label: sphx_glr_auto_examples_linear_model_plot_sparse_recovery.py (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/generated/sklearn.mixture.VBGMM.rst:22: WARNING: undefined label: vbgmm (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/model_selection/_search.py:docstring of sklearn.model_selection.GridSearchCV:46: WARNING: undefined label: multivalued_scorer_wrapping (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/model_selection/_search.py:docstring of sklearn.model_selection.RandomizedSearchCV:61: WARNING: undefined label: multivalued_scorer_wrapping (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/model_selection/_validation.py:docstring of sklearn.model_selection.cross_validate:42: WARNING: undefined label: multivalued_scorer_wrapping (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/model_selection/_validation.py:docstring of sklearn.model_selection.permutation_test_score:38: WARNING: undefined label: _scoring_parameter (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/model_selection/_validation.py:docstring of sklearn.model_selection.permutation_test_score:38: WARNING: undefined label: _scoring (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/generated/sklearn.neighbors.LSHForest.rst:21: WARNING: undefined label: approximate_nearest_neighbors (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/sklearn/neighbors/lof.py:docstring of sklearn.neighbors.LocalOutlierFactor:78: WARNING: undefined label: sklearn.metrics.pairwise.pairwise_distances (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/grid_search.rst:87: WARNING: undefined label: sphx_glr_auto_examples_model_selection_plot_multi_metric_evaluation (if the link has no caption the label must precede a section header)
/home/ubuntu/scikit-learn/doc/modules/grid_search.rst:186: WARNING: undefined label: sphx_glr_auto_examples_model_selection_plot_multi_metric_evaluation (if the link has no caption the label must precede a section header)