How to integrate charts in ireports

I have to create progress report .In that comparison bar graph between two term examination  i have to create.I have taken new dataset  for the chart query.

My database table is  like this

student id     examgroup   marks  subjectid

1                     1               85             1

1                       1             98              2

1                     2                85             1

1                    2               65              2

when am creating chart its displaying only one exam group graph.can anybody suggest how to create comparison graph between two terms