Function stacksFromColumns

Build a StackedBarSeries from a wide series — one numeric column per stack group. This is the reader for pond's pivotByGroup output (long → wide reshape: each group value becomes its own column), or any series that is already wide (e.g. in / out traffic). columns names the segment columns bottom → top; a ValueSeries bins on its value axis (neighbour-spaced slots), a TimeSeries on its key (interval spans or neighbour-spaced points).