Thank you for contacting Aspose support.
We have tried to manually achieve your requirement using MS Excel application. MS Excel does not sort the data as you required it. With Excel, when data is sorted from smallest to largest, only then the cells containing the dash (-) appears at the last of the list. In case the data is sorted from largest to smallest, the dash appears at the start of the list. Aspose.Cells APIs follow MS Excel standards & recommendations therefore your goals cannot be achieved with Aspose.Cells API.
In case you can find a way to sort the data as per your requirement using MS Excel then please share the steps to achieve it, so we may try to mimic the functionality with Aspose.Cells API or log an appropriate feature request for the future releases.