It would really interesting to have a plot for date or year a book was bought vs the year it was read. To have an idea of how long it took to read it. This would be particularly interesting for people who tend to buy a lot but not read as much, having a visual evidence of how much they are buying each year vs how much they are reading each year would be really cool.

Obviously this would require a "buy day" field that readers would have to fill in, but I don't think that's too hard to add.