>>96232311
I found some stats for you, but not the pretty graph you wanted showing percent books published independently, sorry.

https://wordsrated.com/book-publishing-companies-statistics/

>In the worldwide book industry, there were approximately 69,709 independent publishers as of 2020.
The region of North American has a 35% market share in book publishing, greatly contributed to by indie publishers.

>Out of the commercially available books, independent books make up around 40%.
For independent publishers, securing distribution (39%), returns (35%) and understanding distribution fees (27%) made up their biggest concerns as of 2022.

>As of 2021, the US indie book publishers with the fastest growth were Microcosm Publishing (1st), Blue Star Press (2nd) and Media Lab Books (3rd).

>A graph is given below to show the number of new titles by the fastest growing US indie book publishers: