Could you please clarify (a screenshot perhaps) what is the slight difference from the actual position?
Are you using piecharts on shapes? In that case the coordinates are calculated automatically based on the shape polygon and there isn't a list of coordinates to use for that.
You could also show the piecharts on top of country coordinates that you set via layer with type
items. This would both allow you to use automatic proximity aggregation and also would give you known coordinates for each country.
One potential coordinate source you could use can be downloaded from https://apps.zoomcharts.com/spreadsheet-analytics/services/geo/Countries.ashx. The data is from geonames.org (distributed under Creative Commons Attribution 3.0 License).