Melancholy is virtually non-existent before 1570, but begins to rise and then falls until it drops off completely around 1625, about the time of the death of Dowland. It builds again to a great surge in 1650 (when, it says in Wikipedia, ‘the Age of Discovery ends’: reason enough), falls and then picks up, growing nicely and rising with the Romantics in 1800, and then declines gently before starting to increase again after 2000. Sting recorded a very terrible version of Dowland’s songs in 2006. [Ed. !!] Fuck is quite absent from books until about 1590 when it jolts up the chart for about eight years and then plummets, before returning in the 1630s...
To see what's really going on one should run the search without smoothing:
So it's mostly just a signal-to-noise artifact that's due to small corpora. (Here's the post-1750 part expanded.) It is not credible that "fuck" was absent until the 1590s; late medievals and especially Scots -- Dunbar, Lyndsay, etc. -- were fond of the word, which for some reason was preferred in the north to "swive."
By the way, the gem-studded OED entry for "fuck" has this beautiful inadvertent alexandrine by Florio, defining "fottere" in World of Words:
to iape, to sard, to fucke, to swive, to occupy.
(Note to self/others: use in parody of Tennyson's "Ulysses.")
Another n-gram I wanted to post, although I don't have the time to discuss it in detail, is the "brow" words:
As you can see they're all recent, not just middlebrow. All of them are coeval -- middlebrow perhaps exactly contemporary -- with T.S. Eliot's "dissociation of sensibility" thesis. I suspect that the dissociation of brows had something to do with this. In particular there's a Church of Latter-Day Middlebrows that the modernists came up with, beginning with Dryden (I remember J.D. McClatchy actually calling Dryden a middlebrow) and running through Addison, Richardson, Johnson (but not Pope or Swift, who were playful and mad respectively), and Walter Scott, to the high Victorian period when the distinction between middlebrows and highbrows becomes imperceptible. This lines up nicely with Eliot's history of English poetry, in which all poetry after Milton and Dryden (until Eliot, of course) was either ethereal and goofy or businesslike and drab.
Update Jenny Davidson asks in comments about the "standard" history of highbrow, which is presumably a phrenology thing although I can't locate any reliable early sources of an explicitly phrenological character. Assuming that this is so, it's interesting that even highbrow dates from the later days of phrenology. Perhaps this is because the terms had no real, non-jocular application before the era of Bloomsbury and Eliot.