Where do you import it from? And how it is set up there?
If you're using Excel or Access, Tableau will look at the first 10 rows of data to determine the data type. Therefore, if the X value is not in these first 10 rows, Tableau will recognize it as a number. This can be fixed by forcing it to a text field within Excel. Another option would be to create a schema .ini file. http://kb.tableausoftware.com/articles/knowledgebase/displaying-data-types-correctly
Hope this helps!
This makes sense and is very helpful - I'll have a go and see what happens.
Thanks very much!