My logs are full of bot visitors, often from Eastern Europe and China.
The bots are identified as Ahrefs, Seznam, LSSRocketCrawler, Yandex, Sogou and so on.
Should I block these bots from my site and why?

Which ones have a legitimate purpose in increasing traffic to my site?
Many of them are SEO.

I have to say I see less traffic if anything since the bots have arrived in large numbers.

It would not be too hard to block these since they all admit in their User Agent that they are bots.


Question: Placing the grid along date tickmarks
Answer: The function axis draws your axes, tick marks and labels, and returns the tick mark positions as a vector.

Since you have Date data, you need to use axis.Date to do this, and then use abline to plot the grid:

z=data.frame(
  x=seq(as.Date("2001-01-01"), by="+1 month", length.out=10)
  y=1:10
)
plot(y~x, data=z)
abline(v=axis.Date(1, z$x), col="grey80")