Hi Elise,

you need to change the chart type for your scatter plots from line to XY scatter in order to accurately modify their X-position.
I quickly did that and attached your sample again. The correct X-position is dependant on the width of your columns.
In your sample X-pos 0.85 is the center of the blue column, 1 is the center between the blue and the green, 1.15 is the center of the green one...

If you need more chart samples, have a look over
here http://www.pimpmychart.com

Regards

Holger