array with 1 feature.

y : array-like of shape (n_samples,)
    Training target.

sample_weight : array-like of shape (n_samples,), default=None
    Weights. If set to None, all weights will be set to 1 (equal
    weights).

Returns
-------
self : object
    Returns an instance of self.

Notes
-----
X is stored for future use, as :meth:`transform` needs X to interpolate
new input data.
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