The stats on your main promotion page show the overall, or combined stats for all mailings of that single promotion, but you can still view stats for all mailings separately. This works from any Compare Stats page, too.
Click on any stat type, like engaged, to view details per mailing.
To view stats and the pie graph for a single send, click the stats button next to that mailing.
You can also click the promotion bar in the visual display.
The pie graph appears on the next page. Toggle back and forth as needed!