However, I can't find a way to import already aggregated data. For instance, I have a spreadsheet that has in each row some columns that are dimension values and some columns that are measures. I.e. a projection of a data cube where each row corresponds to a cell.
Why do I want to do this? Why not import the disaggregated data and do the aggregation in Tableau?
Sometimes I don't have the disaggregated data. E.g. published data like census summary statistics where basically the underlying hypercube is published as a series of cross-tabulations in Excel, where each workbook or worksheet is slice of the cube. (In this situation sometimes there is a cube cell per Excel cell and the row and column labels are items in two of the cube dimensions).
Sometimes the aggregation operation is not supported in Tableau, e.g. I want to import the results of statistical tests on the disaggregated data and use Tableau to organise and publish them.
So far as I can see, I can import only raw disaggregated data or aggregate data in esoteric proprietary formats like Essbase. But maybe I'm not looking carefullly enough into the functions of Tableau.