A tuple containing the plots belonging to a slice.
A 2-element tuple representing a slice. The first element is the key
of a feature (ex: 'color'), and the second element is the value (ex:
'green'). An empty tuple represents an 'overall' slice (i.e. one that
encompasses the entire dataset.
A dict mapping
sub_key_id to plot data. The data
contains histograms and confusion matrices, which can be rendered with