How can I add a line to one of the facets?

Another way to express this which is possibly easier to generalize (and formatting stuff left out):

ggplot(df, aes(x,y)) +
  geom_point() + 
  facet_wrap(~ z) +
  geom_vline(data = subset(df, z == "b"), aes(xintercept = 1))

The key things being: facet first, then decorate facets by subsetting the original data frame, and put the details in a new aes if possible. Other examples of a similar idea:

ggplot(df, aes(x,y)) +
  geom_point() + 
  facet_wrap(~ z) +
  geom_vline(data = subset(df, z == "b"), aes(xintercept = 1)) + 
  geom_smooth(data = subset(df, z == "c"), aes(x, y), method = lm, se = FALSE) +
  geom_text(data = subset(df, z == "d"), aes(x = -2, y=0, label = "Foobar"))

I don't have your data, so I made some up:

df <- data.frame(x=rnorm(100),y=rnorm(100),z=rep(letters[1:4],each=25))

ggplot(df,aes(x,y)) +
  geom_point() +
  theme_bw() +
  facet_wrap(~z)

To add a vertical line at x = 1 we can use geom_vline() with a dataframe that has the same faceting variable (in my case z='b', but yours will be levels='major'):

ggplot(df,aes(x,y)) +
  geom_point() +
  theme_bw() +
  facet_wrap(~z) +
  geom_vline(data = data.frame(xint=1,z="b"), aes(xintercept = xint), linetype = "dotted")