How are daily likes and comments calculated?

The daily number of likes and comments is related to the date on which your media was posted and not the date on which the likes and comments were given. 
​
This means that if you post a media on May 1st and if a like (or a comment) is given on May 2nd on this specific media, the like will be dated May 1st on likes and comments history charts. 
​
That's why you may have 0 likes and comments for some days, it's (probably) because you didn't post any media on these specific days.