Melissa,
Excel expects the data on the left to be the x values, as Jon explained. If
you don't want to move the data itself you can probably just right click the
chart, select source data, select the series you want to alter, and select
the data for both the x and y values manually (click the button next to the
field and it will minimize, and then just click and drag to select the data
you want).
Hope this helps.
Jeff