This is likely being caused by the "size to data" being checked on your layout tab of your chart. If your chart width is 500 pixels on your caption tab and you only have 450 pixels used in your chart. Size to data will reduce the width of the image on the screen. However, I am pretty sure that Nprinting imports the actual width of the chart from the size.
So, to resolve this I disable size to data and then adjust the chart width and height on the caption tab to exactly the size of the object.
This should work for you unless your chart can grow substantially.
Try that and see if it helps
what Mike recommended is a good one. There is also an article and part of it uses powerpoint as an example, which may help you as well
FYI, if you wind up doing something similar in word, I sugest you build a table in word and then place the objects in the table, align as desired and then hide the grid lines - this will help avoid the same issue you are having in PowerPoint