Q:

Inner ticks in ggplot

I want to add inner ticks to my plot given by a vector. 
say my vector is myvec <- c(1,3,4:9, 12, 15)
and my plot:
library(ggplot2)
df <- data.frame(x=seq(1:100), y=sort(rexp(100, 2), decreasing = T))

ggplot(df, aes(x=x, y=y)) + geom_point() + 
 scale_y_continuous(limits = c(0, 4))

I now want to add inside facing ticks at x= myvec, y=0 in blue color. How do I do it? 
Tried to work with this solution, but could not use the vector.
Annotate ggplot with an extra tick and label

A:

Try This:
df2<- cbind.data.frame("myvec" = myvec, z= rep(0, length(myvec)))

ggplot(df, aes(x=x, y=y)) + geom_point() + 
  scale_y_continuous(limits = c(0, 4)) +
  geom_point(data=df2, aes(x=myvec, y=z), shape = "\U2714", color = "blue", size= 2)

