You can't mix DateAxis (a specialization of ValueAxis) with CategoryAxis. If you want to do something like
Your only choice is to define your parent category with the name of the category and then add subcategories with customized labels in order to show formatted dates or whatever you want.
If you want to write dates use this ( http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html) to format the string
See the example here: