Graph help

I need some help with Numbers. I'm trying to teach my kids how to get the most out of their computers. One of them has to make a graph so I thought, yay! We can try Numbers! (Then I discovered we can do it in Pages as well!)

Anyway, why are the bars in my graph not centered over the year? I tried following the tutorial on the Apple website, their's are centered of course, but I just can't get it to work out. Can anyone tell me what I'm doing wrong?