Dimensions with linking for ticks and labels

[achilleas-k]

Dimensions can link to either a DataArray or a DataFrame for ticks or labels (depending on Dimension type).

DataArray links

The link object should hold a series of indices that indicate a single vector to represent the ticks or labels. The dimension which holds the relevant values should be indicated by -1.

DataFrame links

The link object should hold a single index which defines the Column to represent the ticks or labels.

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This replaces AliasRangeDimension, which becomes a RangeDimension that links to the DataArray that holds it, and DataFrameDimension, which becomes a SetDimension (or any dimension now) that links to a DataFrame.