Line Graph: Treating multiple entries for the same date as a sum?

In Excel '07, I need to make a line graph showing total hours worked by multiple people in the same day. Right now, it's showing each individual value (for example, on 10/18 one person worked 1 hour and the other worked 2, and it's showing a 1 and a 2 side by side for that day as spikes in the graph), and I want it to show the total (on that day, it should show a 3 for the day, the total for both people).
How do I tell Excel to treat multiple entries for the same day as a sum?

Thanks!!

From what I've been able to learn, Excel's charts are not really designed for data processing. The best way I've found to do the kind of thing you want is to build a table in the spreadsheet that will sum up the things you want, then use that data for the chart.