Creating Charts in Jaspersoft Studio

I am using Jaspersoft Studio for creating charts and exporting purpose. I am passing data to the report through bean instead of database.

So for creating a chart, i have two Array Lists, one for category and one for its respective count. When i use arraylist in the category expression, it gives a casting error as "java.lang.ClassCastException: java.util.ArrayList cannot java.lang.Comparable.

So can anyone help me for solving this or suggesting an alternative to create charts through Java ArrayLists?