if you're not familiar with crosstab, as previously suggested, you could create a subreport just to show the column totals and put it "before" the detail band (for example in the page header band).
obviously, following this way, you will access the datasource twice, and this could could be something you want to avoid, especially if time matters.
basically, a crosstab is the better solution overall, but if you need something simpler (maybe you're not familiar with iReport) or one-shot-like you could think about a subreport
