Order of fields in buckets

I just started developing a new custom visualisation. How do I get the field order in a bucket?   

I have a bucket named "Values" and let's say that I drag "Vente$", and then "Cost$".

The order of the DataViewCategoricalColumn s in dataView.categorical.values are Ventes$, first, Cost$ second as expected.

Now, if I drag Cost$, above Vente$ in the Valeurs bucket...

the dataView.categorical.values I get is exactly the same, in the same order (seems like the order in which the fields were added first to the bucket), regardless of what is the current fields order in the bucket. 

I know that other visualistion (like the table for exemple), the order of the displayed columns change when you reorder the field in the bucket.  So, where can I get the field bucket index? 

dataRoles definition;

{
  name: 'Values',
  kind: VisualDataRoleKind.GroupingOrMeasure,
}, {
  name: 'Gradient',
  kind: VisualDataRoleKind.Measure,
}

dataViewMappings defintion;

values: {                   
    select: [ { bind: { to: 'Values' } },{ bind: { to: 'Gradient' } }],
    dataReductionAlgorithm: { top: {} }
},