Fathom itself has no command for reversing rows and columns. But if your data is in Excel (or if you export your data and read it into Excel) you can use it to accomplish the reversal.
First you select the data, then copy it, then select a blank cell somewhere and choose Paste Special from the Edit menu. In the resulting dialog box, check the Transpose option and click OK.
Now, copy the data from Excel and paste it into an empty collection in Fathom.