These numbers I don't think tell the real story about how individual publishers are doing. Looking at the bookscan numbers from 2017 you see a major shift in the book market. That image indicates that the book market has been growing, which it has been, but looking at individual publishers in 2017 DC saw a huge collapse in its GN sales.
Year - Units - Sales
2015 - 1,074,304 - $21,701,088
2016 - 1,234,047 - $23,203,071
2017 - 827,544 - $15,234,525
In 2017 DC lost nearly a 3rd of its total sales from the year before in GNs. Image and Marvel had similar situations.