David, I'm not sure I'm following you. The graph displays both the total number of magazines Bukowski was in and the total number of magazine issues with his work. Both lines represent his periodical appearances in the 1950-70 period. I'm not comparing his appearances to anyone else's... yet...
It began right after Jane's death, and it took him a while to completely recover.
In the Burning in Water intro he says he was in "a slump or a blackout" after the publication of It Catches at the end of 1963, and that the Webbs brought him to New Orleans in early 1965 to write for Crucifix. I would hazard a guess that the "slump" was at least partially brought about by Frances being pregnant, and everything that went along with that. They would have known she was pregnant in January or February of 1964, since Marina was born at the beginning of September. Which would make a three year "dip" if you include any extended mourning over Jane.The 62-63 dip was mostly caused by the "depression". Marina was not born yet, and the Webbs used old poems in It Catches.
I'm not doubting your research, I'm just saying that publication is not necessarily an accurate indicator of whether he was writing. It's speculation to apply publication numbers to anything other than publication.Those are the facts.