@sh.add_function(dsp, outputs=['labels'], inputs_kwargs=True,
inputs_defaults=True)
def define_labels(x_label='x', y_label='y'):
"""
Defines `reference-labels` (i.e., "x", "y") for each data-set.
:param x_label:
Default `var-name` of the common x-axis to synchronise and re-sampled
the data-sets.
:type x_label: str
:param y_label:
Default `var-name` of the common y-axis to synchronise the data-sets.
:type y_label: str
:return:
Reference-labels (i.e., "x", "y") for each data-set.
It is like `{"<set-name>": {"x": "<x-label>", "y": "<y-label>"}, ...}`.
:rtype: collections.defaultdict
"""
import collections
return collections.defaultdict(lambda: dict(x=x_label, y=y_label))