General info on how to use the 3D Charting API can be found under '3D numerical chart'.
A categorical representation is only possible for grid-based data. In terms of the API this means any class deriving WAbstractGridData. Wt already provides two implementations: WGridData and WEquidistantGridData. To create a categorical chart, the chart must be configured as such (by setting the type to CategoryChart) and the data which is added to this chart must be of the type BarSeries3D.
A categorical chart also allows multiple dataseries to be shown if the dimensions are the same. The different bar-series are then stacked on top of each other.