This is calculated as the sum of all likes, comments and saves divided by the amount of Reach that your post received. The graph shows the average Engagement Rate by Reach of posts published on the given day.
Engagement Rate by Reach can also be found for posts in the 'Top posts by' area at the bottom of the 'Posts' page:
Historical data for Engagement Rate by Reach is available for Instagram business accounts up to two years, or since converted to a business account.