I'll take a look over the weekend sometime and see what is out there, I hate getting in your way for stuff like this as know you have your own never-ending todo list. Appreciate the offer for sure, may well end up taking you up on that if it gestates beyond the current random idea stage.
If you or anyone has any suggestions about features or how it would work etc please feel free to chip in, it might help motivate me to pull it together. I have given no thought as yet as to how it might work etc. I just figure given many calibre users spend hours collecting minutae of data about their books and reading, that giving them some additional pretty output might be of interest. Certainly the stats pages on goodreads showing me how many pages I have read each year, how many books each year etc I find interesting, particularly if you happen to set yourself some goals for a bit of fun.
Clearly there will need to be some level of customisation allowing users to setup their own desired graphs, though where these will be rendered (modeless window) and what it all looks like is all up for grabs... ideas welcomed.