Maybe a reference line in the Pages/Books read chart that would basically show you the monthly pace.
E.g., You have an annual page challenge of 36000, then there would be a line at the 3000 pages axis mark.
Similar for book count - you want to read 60 books a year, then there would be a reference line at 5 books axis mark.
That way you could visually track your pace - see how much below/above you are for your "monthly target".

Big +1, I would love this. I calculate this manually throughout the year and having to google "what day number is it" all the time is a bummer 😂 I'd love it on the pages and hours stats too, since I do value all three of those, otherwise I'd never read long books.