how to set ranges for values in x-axis for a chart in jasper reports

[kiran_3]

i have table name say 'call_records' having sample data as follows

id	5
member_id	542
call_duration	97
created	2014-11-28 17:00:28
status	answered

here call_duration is varchar(255) i want get the call_duration time and show in a chart like , on x-axis points(call_duration) should be like 0-20 , 21-40, 41-60 and so on and each point will have a bar chart(if i took as bar chart while designing) having height of its count(ex:number of people who spoke between 0-20). how can i achieve this in jasper reprots, i can't really frame the question to search in google but even though i tried didn't get the expecetd solution . i am using jaspesoft studio.here is the query i am using to get call duration for every record.

select vmc.created as 'date', vmc.call_duration as 'call_duration' from call_records as vmc where vmc.created between $P{startDate} AND $P{endDate}[/code]