For custom pie charts, I'd love to be able to have a slice that includes all the books in the total that are not tagged with whatever tags inform the other slices.

For instance: I use tags to track which books were recommended to me by whom. And one of my custom pie charts shows me those figures. But I'd like to see what percentage of the books were recommended (and by whom) against the total number of books from that time period. Currently the total books in that pie chart is the sum of books tagged with those "recommended" tags. It would also be interesting to do a comparison chart of recommendations in my books read and in my tbr. I hope this makes sense.